Ab Initio Studies of Stratospheric Ozone Depletion Chemistry
Lee, Timothy J.; Head-Gordon, Martin; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
An overview of the current understanding of ozone depletion chemistry, particularly with regards the formation of the so-called Antarctic ozone hole, will be presented together with an outline as to how ab initio quantum chemistry can be used to further our understanding of stratospheric chemistry. The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of fluorine, chlorine, bromine and nitrogen oxide species will be demonstrated by presentation of some example studies. The ab initio results will be shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the theoretical results are shown to fill in the gaps and to resolve experimental controversies. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of halogen oxide species will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of experimental studies.
Ab-initio study of thermoelectricity of layered tellurium compounds
Ibarra Hernández, Wilfredo
2015-01-01
In this thesis, we explore the electronic, dynamic and thermoelectric properties of different tellurium-based compounds. We perform ab-initio calculations within the Vienna Ab-initio Simulation Package (VASP) that works in the framework of Density Functional Theory (DFT). For the thermoelectric properties, we use the Boltztrap code that solves the Boltzmann Transport Equations (BTE) for electrons within the Constant Relaxation Time Approximation (CRTA). This computational pa...
Ab-initio study of napthelene based conducting polymer
Ruhela, Ankur [Advanced Materials Research Group, Computational Nanoscience and Technology Lab (CNTL), ABV-Indian Institute of Information Technology and Management, Gwalior -474010, India and Amity Institute of Nanotechnology, Amity University, Noida-201303 (India); Kanchan, Reena, E-mail: reena.kanchan1977@gmail.com [Department of Chemistry, Jiwaji University, Gwalior-474001 (India); Srivastava, Anurag [Advanced Materials Research Group, Computational Nanoscience and Technology Lab (CNTL), ABV-Indian Institute of Information Technology and Management, Gwalior -474010 (India); Sinha, O. P. [Amity Institute of Nanotechnology, Amity University, Noida-201303 (India)
2014-04-24
In this paper, we have identified structural and electronic properties of conducting polymers by using DFT based ATK-VNL ab-initio tool. Naphthalene derivative structures were stabilized by varying the bond length between two atoms of the molecule C-N and C-C. We have also studied the molecular energy spectrum of naphthalene derivatives and found the HOMOLUMO for the same. A comparison of structural and electronic properties of naphthalene derivatives by attaching the functional group of amine, have been performed and found that they show good semi conducting properties.
Ab-initio study of napthelene based conducting polymer
In this paper, we have identified structural and electronic properties of conducting polymers by using DFT based ATK-VNL ab-initio tool. Naphthalene derivative structures were stabilized by varying the bond length between two atoms of the molecule C-N and C-C. We have also studied the molecular energy spectrum of naphthalene derivatives and found the HOMOLUMO for the same. A comparison of structural and electronic properties of naphthalene derivatives by attaching the functional group of amine, have been performed and found that they show good semi conducting properties
Relaxation of Small Molecules: an ab initio Study
CAO Yi-Gang; JIAO Zheng-Kuan; A. Antons; K. Schroeder; S. Blügel2
2002-01-01
Using an ab initio total energy and force method, we have relaxed several group IV and group V elementalclusters, in detail the arsenic and antimony dimers, silicon, phosphorus, arsenic and antimony tetramers. The obtainedbond lengths and cohesive energies are more accurate than other calculating methods, and in excellent agreement withthe experimental results.
Ab initio study of phase equilibria in TiCx
Korzhavyi, P.A.; Pourovskii, L.V.; Hugosson, H.W.;
2002-01-01
The phase diagram for the vacancy-ordered structures in the substoichiometric TiCx (x = 0.5-1.0) has been established from Monte Carlo simulations with the long-range pair and multisite effective interactions obtained from ab initio calculations. Three ordered superstructures of vacancies (Ti2C, ...
Molecular ion LiHe+: ab initio study
Highlights: ► Excited electronic states of LiHe+ are studied. ► Potential energy curves of thirteen states are calculated. ► Dipole moment and transition dipole moment functions are determined. ► Basic spectroscopic properties of the electronic states are derived. - Abstract: High level ab initio calculations are performed on the molecular ion LiHe+. Potential energy curves for the low-lying singlet and triplet electronic states are calculated using the multi-reference configuration interaction and single-reference coupled cluster methods with large basis sets. The corresponding dipole moments and transition dipole moments functions are also determined. The basic spectroscopic properties and excitation energies of the electronic states are derived from rovibrational bound state calculations.
Serine Proteases an Ab Initio Molecular Dynamics Study
De Santis, L
1999-01-01
In serine proteases (SP's), the H-bond between His-57 and Asp-102, and that between Gly-193 and the transition state intermediate play a crucial role for enzymatic function. To shed light on the nature of these interactions, we have carried out ab initio molecular dynamics simulations on complexes representing adducts between the reaction intermediate and elastase (one protein belonging to the SP family). Our calculations indicate the presence of a low--barrier H-bond between His-57 and Asp-102, in complete agreement with NMR experiments on enzyme--transition state analog complexes. Comparison with an ab initio molecular dynamics simulation on a model of the substrate--enzyme adduct indicates that the Gly-193--induced strong stabilization of the intermediate is accomplished by charge/dipole interactions and not by H-bonding as previously suggested. Inclusion of the protein electric field in the calculations does not affect significantly the charge distribution.
Experimental, ab initio and density functional theory studies on sulfadiazine
Ogruc-Ildiz, Gulce; Akyuz, Sevim; Ozel, Aysen E.
2009-04-01
In the present study, combined experimental and computational study on molecular vibrations of free sulfadiazine has been reported. The theoretically possible stable conformers of free sulfadiazine molecule in electronically ground state were searched by means of torsion potential energy surfaces scan studies through C1 sbnd C7 sbnd S8 sbnd N8, C7 sbnd S8 sbnd N9 sbnd C10 and S8 sbnd N9 sbnd C10 sbnd N11 dihedral angles, at both semi-empirical PM3 and B3LYP/3-21G levels of theory. The final geometrical parameters for the obtained stable conformers were determined by means of geometry optimization carried out at ab initio HF/6-31G++(d,p) and DFT/B3LYP/6-31G++(d,p) theory levels. The harmonic and anharmonic vibrational wavenumbers and IR intensities were calculated at the same theory levels used in geometry optimization. The modes of the fundamental vibrations were characterized depending on their the total energy distribution (TED%). In order to fit the calculated harmonic wavenumbers to experimental ones, dual scale factors were used. The experimental infrared and Raman spectra of sulfadiazine in solid phase have been measured and compared with the calculated vibrational spectra of each conformer.
Ab initio study of optical excitations in VO2
Coulter, John; Gali, Adam; Manousakis, Efstratios
2014-03-01
Motivated by recent experimental efforts to fabricate p-n junctions from transition metal oxides (TMOs) and a recent theoretical study claiming TMOs to be good absorbers and promising materials for efficient carrier multiplication, we study the optical properties of a prototypical TMO, the insulator M1 phase of vanadium dioxide (VO2), by ab initio methods. We applied the Bethe-Salpeter equations (BSE) to calculate the optical properties, starting from self-consistent GW quasi-particle energy levels and states. In contrast to expectations, the exciton binding energy obtained by BSE is in good agreement with the experiment. We find that the electron-electron interaction is very strong which makes this material promising for efficient carrier multiplication that might lead to an enhanced efficiency in photo-voltaics applications. To illustrate this more quantitatively, we calculated the impact ionization rate within the independent quasiparticle approximation, and find that the rate is significantly higher than silicon in the region of highest solar intensity, due to the strong multiple carrier excitations.
Ab initio study of neutron drops with chiral Hamiltonians
H.D. Potter
2014-12-01
Full Text Available We report ab initio calculations for neutron drops in a 10 MeV external harmonic-oscillator trap using chiral nucleon–nucleon plus three-nucleon interactions. We present total binding energies, internal energies, radii and odd–even energy differences for neutron numbers N=2–18 using the no-core shell model with and without importance truncation. Furthermore, we present total binding energies for N=8,16,20,28,40,50 obtained in a coupled-cluster approach. Comparisons with quantum Monte Carlo results, where available, using Argonne v8′ with three-nucleon interactions reveal important dependences on the chosen Hamiltonian.
Tailoring magnetoresistance at the atomic level: An ab initio study
Tao, Kun
2012-01-05
The possibility of manipulating the tunneling magnetoresistance (TMR) of antiferromagnetic nanostructures is predicted in the framework of ab initio calculations. By the example of a junction composed of an antiferromagnetic dimer and a spin-polarized scanning tunneling microscopy tip we show that the TMR can be tuned and even reversed in sign by lateral and vertical movements of the tip. Moreover, our finite-bias calculations demonstrate that the magnitude and the sign of the TMR can also be tuned by an external voltage. © 2012 American Physical Society.
Ab initio study of neutron drops with chiral Hamiltonians
We report ab initio calculations for neutron drops in a 10 MeV external harmonic-oscillator trap using chiral nucleon–nucleon plus three-nucleon interactions. We present total binding energies, internal energies, radii and odd–even energy differences for neutron numbers N=2–18 using the no-core shell model with and without importance truncation. Furthermore, we present total binding energies for N=8,16,20,28,40,50 obtained in a coupled-cluster approach. Comparisons with quantum Monte Carlo results, where available, using Argonne v8′ with three-nucleon interactions reveal important dependences on the chosen Hamiltonian
Ab initio and kinetic modeling studies of formic acid oxidation
Marshall, Paul; Glarborg, Peter
2015-01-01
A detailed chemical kinetic model for oxidation of formic acid (HOCHO) in flames has been developed, based on theoretical work and data from literature. Ab initio calculations were used to obtain rate coefficients for reactions of HOCHO with H, O, and HO2. Modeling predictions with the mechanism...... on calculations with the kinetic model. Formic acid is consumed mainly by reaction with OH, yielding OCHO, which dissociates rapidly to CO2 + H, and HOCO, which may dissociate to CO + OH or CO2 + H, or react with H, OH, or O2 to form more stable products. The branching fraction of the HOCHO + OH...
Ab Initio Studies of Halogen and Nitrogen Oxide Species of Interest in Stratospheric Chemistry
Lee, Timothy J.; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of fluorine, chlorine, bromine and nitrogen oxide species will be demonstrated by presentation of some example studies. The ab initio results are shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the theoretical results are shown to fill in the gaps and to resolve experimental controversies. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of halogen oxide species will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of experimental studies.
Lithium Insertion In Silicon Nanowires: An ab Initio Study
Zhang, Qianfan
2010-09-08
The ultrahigh specific lithium ion storage capacity of Si nanowires (SiNWs) has been demonstrated recently and has opened up exciting opportunities for energy storage. However, a systematic theoretical study on lithium insertion in SiNWs remains a challenge, and as a result, understanding of the fundamental interaction and microscopic dynamics during lithium insertion is still lacking. This paper focuses on the study of single Li atom insertion into SiNWs with different sizes and axis orientations by using full ab initio calculations. We show that the binding energy of interstitial Li increases as the SiNW diameter grows. The binding energies at different insertion sites, which can be classified as surface, intermediate, and core sites, are quite different. We find that surface sites are energetically the most favorable insertion positions and that intermediate sites are the most unfavorable insertion positions. Compared with the other growth directions, the [110] SiNWs with different diameters always present the highest binding energies on various insertion locations, which indicates that [110] SiNWs are more favorable by Li doping. Furthermore, we study Li diffusion inside SiNWs. The results show that the Li surface diffusion has a much higher chance to occur than the surface to core diffusion, which is consistent with the experimental observation that the Li insertion in SiNWs is layer by layer from surface to inner region. After overcoming a large barrier crossing surface-to-intermediate region, the diffusion toward center has a higher possibility to occur than the inverse process. © 2010 American Chemical Society.
An ab initio study of plutonium oxides surfaces
By means of first-principles calculations, we have studied the atomic structure as well as the thermodynamic stability of various plutonium dioxide surfaces in function of their environment (in terms of oxygen partial pressure and temperature). All these simulations have been performed with the ABINIT code. It is well known that DFT fails to describe correctly plutonium-based materials since 5f electrons in such systems are strongly correlated. In order to go beyond DFT, we have treated PuO2 and β-Pu2O3 in a DFT+U framework. We show that the couple of parameters (U,J) that works well for pure Pu is also well designed for describing ground state (GS) properties of these two oxides. The major improvement with respect with DFT is that we are able to predict an insulating GS in agreement with experiments. The presence of a gap in the DOS (Density of States) of plutonium oxides should play a significant role in the predicted surface reactivity. However, performing DFT+U calculations on surfaces of plutonium oxide from scratch was too ambitious. That is why we decided, as a first step, to study the stability of the (100), (110) and (111) surfaces of PuO2 in a DFT-GGA framework. For each of these orientations, we considered various terminations. These ab initio results have been introduced in a thermodynamic model which allows us to predict the relative stability of the different terminations as a function of temperature and oxygen partial pressure (pO2). We conclude that at room temperature and for pO2∼10 atm., the polar O2-(100) termination is favoured. The stabilization of such a polar stoichiometric surface is surprising and should be confirmed by DFT+U calculations before any final conclusion. (authors)
Ab initio study of II-(VI){sub 2} dichalcogenides
Olsson, P; Vidal, J; Lincot, D, E-mail: polsson@kth.se [Institut de R and D sur l' energie photovoltaique (IRDEP), UMR 7174-EDF-CNRS-ENSCP, 6 quai Watier, 78401 Chatou Cedex (France)
2011-10-12
The structural stabilities of the (Zn,Cd)(S,Se,Te){sub 2} dichalcogenides have been determined ab initio. These compounds are shown to be stable in the pyrite phase, in agreement with available experiments. Structural parameters for the ZnTe{sub 2} pyrite semiconductor compound proposed here are presented. The opto-electronic properties of these dichalcogenide compounds have been calculated using quasiparticle GW theory. Bandgaps, band structures and effective masses are proposed as well as absorption coefficients and refraction indices. The compounds are all indirect semiconductors with very flat conduction band dispersion and high absorption coefficients. The work functions and surface properties are predicted. The Te and Se based compounds could be of interest as absorber materials in photovoltaic applications. (paper)
Ab initio study of the transition-metal carbene cations
李吉海; 冯大诚; 冯圣玉
1999-01-01
The geometries and bonding characteristics of the first-row transition-metal carbene cations MCH2+ were investigated by ab initio molecular orbital theory （HF/LANL2DZ）. All of MCH2+ are coplanar. In the closed shell structures the C bonds to M with double bonds; while in the open shell structures the partial double bonds are formed, because one of the σ and π orbitals is singly occupied. It is mainly the π-type overlap between the 2px orbital of C and 4px, 3dxz, orbitals of M+ that forms the π orbitals. The dissociation energies of C—M bond appear in periodic trend from Sc to Cu. Most of the calculated bond dissociation energies are close to the experimental ones.
Highly anisotropic thermal conductivity of arsenene: An ab initio study
Zeraati, Majid; Vaez Allaei, S. Mehdi; Abdolhosseini Sarsari, I.; Pourfath, Mahdi; Donadio, Davide
2016-02-01
Elemental two-dimensional (2D) materials exhibit intriguing heat transport and phononic properties. Here we have investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb structure of arsenic, using ab initio calculations. Solving the Boltzmann transport equation for phonons, we predict a highly anisotropic thermal conductivity of 30.4 and 7.8 W/mK along the zigzag and armchair directions, respectively, at room temperature. Our calculations reveal that phonons with mean free paths between 20 nm and 1 μ m provide the main contribution to the large thermal conductivity in the zigzag direction; mean free paths of phonons contributing to heat transport in the armchair directions range between 20 and 100 nm. The obtained anisotropic thermal conductivity and feasibility of synthesis, in addition to high electron mobility reported elsewhere, make arsenene a promising material for nanoelectronic applications and thermal management.
Ab initio studies of phoshorene island single electron transistor.
Ray, S J; Venkata Kamalakar, M; Chowdhury, R
2016-05-18
Phosphorene is a newly unveiled two-dimensional crystal with immense potential for nanoelectronic and optoelectronic applications. Its unique electronic structure and two dimensionality also present opportunities for single electron devices. Here we report the behaviour of a single electron transistor (SET) made of a phosphorene island, explored for the first time using ab initio calculations. We find that the band gap and the charging energy decrease monotonically with increasing layer numbers due to weak quantum confinement. When compared to two other novel 2D crystals such as graphene and MoS2, our investigation reveals larger adsorption energies of gas molecules on phosphorene, which indicates better a sensing ability. The calculated charge stability diagrams show distinct changes in the presence of an individual molecule which can be applied to detect the presence of different molecules with sensitivity at a single molecular level. The higher charging energies of the molecules within the SET display operational viability at room temperature, which is promising for possible ultra sensitive detection applications. PMID:27093536
Ab initio studies of phosphorene island single electron transistor
Ray, S. J.; Venkata Kamalakar, M.; Chowdhury, R.
2016-05-01
Phosphorene is a newly unveiled two-dimensional crystal with immense potential for nanoelectronic and optoelectronic applications. Its unique electronic structure and two dimensionality also present opportunities for single electron devices. Here we report the behaviour of a single electron transistor (SET) made of a phosphorene island, explored for the first time using ab initio calculations. We find that the band gap and the charging energy decrease monotonically with increasing layer numbers due to weak quantum confinement. When compared to two other novel 2D crystals such as graphene and MoS2, our investigation reveals larger adsorption energies of gas molecules on phosphorene, which indicates better a sensing ability. The calculated charge stability diagrams show distinct changes in the presence of an individual molecule which can be applied to detect the presence of different molecules with sensitivity at a single molecular level. The higher charging energies of the molecules within the SET display operational viability at room temperature, which is promising for possible ultra sensitive detection applications.
Ab initio study of MoS2 nanotube bundles
Verstraete, Matthieu; Charlier, Jean-Christophe
2003-07-01
Recently, the synthesis of a new phase of MoS2I1/3 stoichiometry was reported [M. Remskar, A. Mrzel, Z. Skraba, A. Jesih, M. Ceh, J. Demšar, P. Stadelmann, F. Lévy, and D. Mihailovic, Science 292, 479 (2001)]. Electron microscope images and diffraction data were interpreted to indicate bundles of sub-nanometer-diameter single-wall MoS2 nanotubes. After experimental characterization, the structure was attributed to an assembly of “armchair” nanotubes with interstitial iodine. Using first-principles total-energy calculations, bundles of MoS2 nanotubes with different topologies and stoichiometries are investigated. All of the systems are strongly metallic. Configurations with “zigzag” structures are found to be more stable energetically than the “armchair” ones, though all of the structures have similar stabilities. After relaxation, there remain several candidates which give a lattice parameter in relative agreement with experiment. Further, spin-polarized calculations indicate that a structure with armchair tubes iodine atoms in their center acquires a very large spontaneous magnetic moment of 12μB, while the other structures are nonmagnetic. Our ab initio calculations show that in most of the other structures, the tubes are very strongly bound together, and that the compounds should be considered as a crystal, rather than as a bundle of tubes in the habitual sense.
An ab initio study of plutonium oxides surfaces; Etude ab initio des surfaces d'oxydes de Pu
Jomard, G.; Bottin, F.; Amadon, B
2007-07-01
By means of first-principles calculations, we have studied the atomic structure as well as the thermodynamic stability of various plutonium dioxide surfaces in function of their environment (in terms of oxygen partial pressure and temperature). All these simulations have been performed with the ABINIT code. It is well known that DFT fails to describe correctly plutonium-based materials since 5f electrons in such systems are strongly correlated. In order to go beyond DFT, we have treated PuO{sub 2} and {beta}-Pu{sub 2}O{sub 3} in a DFT+U framework. We show that the couple of parameters (U,J) that works well for pure Pu is also well designed for describing ground state (GS) properties of these two oxides. The major improvement with respect with DFT is that we are able to predict an insulating GS in agreement with experiments. The presence of a gap in the DOS (Density of States) of plutonium oxides should play a significant role in the predicted surface reactivity. However, performing DFT+U calculations on surfaces of plutonium oxide from scratch was too ambitious. That is why we decided, as a first step, to study the stability of the (100), (110) and (111) surfaces of PuO{sub 2} in a DFT-GGA framework. For each of these orientations, we considered various terminations. These ab initio results have been introduced in a thermodynamic model which allows us to predict the relative stability of the different terminations as a function of temperature and oxygen partial pressure (p{sub O{sub 2}}). We conclude that at room temperature and for p{sub O{sub 2}}{approx}10 atm., the polar O{sub 2}-(100) termination is favoured. The stabilization of such a polar stoichiometric surface is surprising and should be confirmed by DFT+U calculations before any final conclusion. (authors)
Abild-Pedersen, Frank; Nørskov, Jens Kehlet; Rostrup-Nielsen, Jens;
2006-01-01
Mechanisms and energetics of graphene growth catalyzed by nickel nanoclusters were studied using ab initio density functional theory calculations. It is demonstrated that nickel step-edge sites act as the preferential growth centers for graphene layers on the nickel surface. Carbon is transported...
Lee, Timothy J.; Langhoff, Stephen R. (Technical Monitor)
1995-01-01
The ability of modern state-of-the art ab initio quantum chemical techniques to characterize reliably the gas-phase molecular structure, vibrational spectrum, electronic spectrum, and thermal stability of chlorine oxide and nitrogen oxide species will be demonstrated by presentation of some example studies. In particular the geometrical structures, vibrational spectra, and heats of formation Of ClNO2, CisClONO, and trans-ClONO are shown to be in excellent agreement with the available experimental data, and where the experimental data are either not known or are inconclusive, the ab initio results are shown to fill in the gaps and to resolve the experimental controversy. In addition, ab initio studies in which the electronic spectra and the characterization of excited electronic states of ClONO2, HONO2, ClOOC17 ClOOH, and HOOH will also be presented. Again where available, the ab initio results are compared to experimental observations, and are used to aid in the interpretation of the experimental studies.
Relaxation of Small Molecules：an ab initio Study
CAOYi－Gang; A.Antons; 等
2002-01-01
Using an ab inito total energy and force method,we have relaxed several group IV and group V elemental clusters,in detail the arsenic and antimony dimers,silicon,phosphorus,arsenic and antimony tetraners,The obtained bond lengths and cohesive energies are more accurate than other calculating methods,and in excellent agreement with the experimental results.
Ab initio studies of equations of state and chemical reactions of reactive structural materials
Zaharieva, Roussislava
subject of studies of the shock or thermally induced chemical reactions of the two solids comprising these reactive materials, from first principles, is a relatively new field of study. The published literature on ab initio techniques or quantum mechanics based approaches consists of the ab initio or ab initio-molecular dynamics studies in related fields that contain a solid and a gas. One such study in the literature involves a gas and a solid. This is an investigation of the adsorption of gasses such as carbon monoxide (CO) on Tungsten. The motivation for these studies is to synthesize alternate or synthetic fuel technology by Fischer-Tropsch process. In this thesis these studies are first to establish the procedure for solid-solid reaction and then to extend that to consider the effects of mechanical strain and temperature on the binding energy and chemisorptions of CO on tungsten. Then in this thesis, similar studies are also conducted on the effect of mechanical strain and temperature on the binding energies of Titanium and hydrogen. The motivations are again to understand the method and extend the method to such solid-solid reactions. A second motivation is to seek strained conditions that favor hydrogen storage and strain conditions that release hydrogen easily when needed. Following the establishment of ab initio and ab initio studies of chemical reactions between a solid and a gas, the next step of research is to study thermally induced chemical reaction between two solids (Ni+Al). Thus, specific new studies of the thesis are as follows: (1) Ab initio Studies of Binding energies associated with chemisorption of (a) CO on W surfaces (111, and 100) at elevated temperatures and strains and (b) adsorption of hydrogen in titanium base. (2) Equations of state of mixtures of reactive material structures from ab initio methods. (3) Ab initio studies of the reaction initiation, transition states and reaction products of intermetallic mixtures of (Ni+Al) at elevated
Ab Initio Studies on Hematite Surface and the Adsorption of Phosphate
Swati Chaudhury; Chandrika Varadachari; Kunal Ghosh
2014-01-01
This investigation explores the ab initio DFT method for understanding surface structure of hematite and the nature and energetics of phosphate adsorption. Using the full potential linearized plane wave method (FP-LAPW), we derived the structure and energies of various magnetic forms of hematite. The antiferromagnetic (AFM) form was observed to be the most stable. Hematite surfaces with Fe-termination, O-termination, or OH-termination were studied. The OH-terminated surface was the most stabl...
Ab Initio Study on the Anti-HIV Activities of Hydroxyflavones
ZHANG Yu
2005-01-01
Flavone and 95 hydroxyflavones have been studied with ab initio method, and their total energies, atomic charges, dipole moments, multipole moments, molecular orbital compositions, orbital energies etc. were obtained. Among them the relationship between total atomic charges and activities against HIV is basically in accordance with the experimental results. The beneficial references are provided for the extraction and synthesis of strong active anti-HIV medicines.
Ab initio study on the magnetostructural properties of MnAs
Sanvito, Stefano; RUNGGER, IVAN
2006-01-01
The magnetic and structural properties of MnAs are studied with ab initio methods, and by mapping total energies onto a Heisenberg model. The stability of the different phases is found to depend mainly on the volume and on the amount of magnetic order, confirming previous experimental findings and phenomenological models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stab...
Hydrogen adsorption in ZIF-7: A DFT and ab-initio molecular dynamics study
Dixit, Mudit; Major, Dan Thomas; Pal, Sourav
2016-05-01
Primary H2 adsorption sites in a zeolitic imidazolate framework, ZIF-7, are identified using ab-initio density functional theory (DFT) based molecular dynamics annealing simulations. The simulations suggest several low energy adsorption sites. The effect of light transition metal decoration on hydrogen storage properties was studied. Our ab-intio DFT calculations illustrate that decorating the ZIF with Sc increases both the number of H2 molecules, as well as the H2 binding energy. The binding energy (∼25 kJ/mol per H2) at 8H2 loading in the pore, suggests that Sc-ZIFs can be potential candidates for hydrogen storage.
Liquid Be, Ca and Ba. An orbital-free ab-initio molecular dynamics study
Several static and dynamic properties of liquid beryllium (l-Be), liquid calcium (l-Ca) and liquid barium (l-Ba) near their triple point have been evaluated by the orbital-free ab initio molecular dynamics method (OF-AIMD), where the interaction between valence electrons and ions is described by means of local pseudopotentials. These local pseudopotentials used were constructed through a force-matching process with those obtained from a Kohn-Sham ab initio molecular dynamics study (KS-AIMD) of a reduced system with non-local pseudopotentials. The calculated static structures show good agreement with the available experimental data, including an asymmetric second peak in the structure factor which has been linked to the existence of a marked icosahedral short-range order in the liquid. As for the dynamic properties, we obtain collective density excitations whose associated dispersion relations exhibit a positive dispersion
Study of atomic structure of liquid Hg-In alloys using ab-initio molecular dynamics
Sharma, Nalini; Ahluwalia, P. K. [Department of Physics, Himachal Pradesh University, Shimla(HP)-171005 (India); Thakur, Anil [Department of Physics, Govt. P. G. College Solan (HP)-173212 (India)
2015-05-15
Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Five liquid Hg-In mixtures (Hg{sub 10}In{sub 90}, Hg{sub 30}In{sub 70}, Hg{sub 50}In{sub 50}, Hg{sub 70}In{sub 30} and Hg{sub 90}In{sub 10}) at 299K are considered. The radial distribution function g(r) and structure factor S(q) of considered alloys are compared with respective experimental results for liquid Hg (l-Hg) and (l-In). The radial distribution function g(r) shows the presence of short range order in the systems considered. Smooth curves of Bhatia-Thornton partial structure factors factor shows the presence of liquid state in the considered alloys.
Study of atomic structure of liquid Hg-In alloys using ab-initio molecular dynamics
Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Five liquid Hg-In mixtures (Hg10In90, Hg30In70, Hg50In50, Hg70In30 and Hg90In10) at 299K are considered. The radial distribution function g(r) and structure factor S(q) of considered alloys are compared with respective experimental results for liquid Hg (l-Hg) and (l-In). The radial distribution function g(r) shows the presence of short range order in the systems considered. Smooth curves of Bhatia-Thornton partial structure factors factor shows the presence of liquid state in the considered alloys
Electronic properties of liquid Hg-In alloys : Ab-initio molecular dynamics study
Sharma, Nalini; Thakur, Anil; Ahluwalia, P. K.
2016-05-01
Ab-initio molecular dynamics simulations are performed to study the structural properties of liquid Hg-In alloys. The interatomic interactions are described by ab-initio pseudopotentials given by Troullier and Martins. Three liquid Hg-In alloys (Hg10In90, Hg30In70,. Hg50In50, Hg70In30, and Hg90Pb10) at 299 K are considered. The calculated results for liquid Hg (l-Hg) and lead (l-In) are also drawn. Along with the calculated results of considered five liquid alloys of Hg-In alloy. The results obtained from electronic properties namely total density of state and partial density of states help to find the local arrangement of Hg and In atoms and the presence of liquid state in the considered five alloys.
Ab-initio approach to study hydrogen diffusion in 9Cr steels
We calculate the equilibrium energies and migration barriers of Fe, Cr and H interstitial defects in α-FeX(X=Cr). We use the ab-initio electronic structure code, SIESTA, coupled to the monomer method to find activated states (or migration barriers), in order to study atomic defects diffusion. Ab-initio calculations reveal that in the presence of Cr the H migration barriers are higher than in pure α-Fe. On the other hand, our permeation tests on 9%Cr-91%Fe alloys reveal a permeation coefficient 10 times lower and a diffusion coefficient 200 times lower than in pure, annealed iron. Focusing on our experimental results, we explore very simple model of new H trapping sites and possible migration paths that can explain the experimental observations.
The behaviour of silicon carbide under irradiation has been studied using classical and ab initio simulations, focusing on the nano scale elementary processes. First, we have been interested in the calculation of threshold displacement energies, which are difficult to determine both experimentally and theoretically, and also the associated Frenkel pairs. In the framework of this thesis, we have carried out simulations in classical and ab initio molecular dynamics. For the classical approach, two types of potentials have been used: the Tersoff potential, which led to non satisfactory results, and a new one which has been developed during this thesis. This potential allows a better modelling of SiC under irradiation than most of the empirical potentials available for SiC. It is based on the EDIP potential, initially developed to describe defects in silicon, that we have generalized to SiC. For the ab initio approach, the feasibility of the calculations has been validated and average energies of 19 eV for the C and 38 eV for the Si sublattices have been determined, close to the values empirically used in the fusion community. The results obtained with the new potential EDIP are globally in agreement with those values. Finally, the elementary processes involved in the crystal recovery have been studied by calculating the stability of the created Frenkel pairs and determining possible recombination mechanisms with the nudged elastic band method. (author)
Ab Initio Studies of Shock-Induced Chemical Reactions of Inter-Metallics
Zaharieva, Roussislava; Hanagud, Sathya
2009-06-01
Shock-induced and shock assisted chemical reactions of intermetallic mixtures are studied by many researchers, using both experimental and theoretical techniques. The theoretical studies are primarily at continuum scales. The model frameworks include mixture theories and meso-scale models of grains of porous mixtures. The reaction models vary from equilibrium thermodynamic model to several non-equilibrium thermodynamic models. The shock-effects are primarily studied using appropriate conservation equations and numerical techniques to integrate the equations. All these models require material constants from experiments and estimates of transition states. Thus, the objective of this paper is to present studies based on ab initio techniques. The ab inito studies, to date, use ab inito molecular dynamics. This paper presents a study that uses shock pressures, and associated temperatures as starting variables. Then intermetallic mixtures are modeled as slabs. The required shock stresses are created by straining the lattice. Then, ab initio binding energy calculations are used to examine the stability of the reactions. Binding energies are obtained for different strain components super imposed on uniform compression and finite temperatures. Then, vibrational frequencies and nudge elastic band techniques are used to study reactivity and transition states. Examples include Ni and Al.
Ab initio studies of magnetism in the iron chalcogenides FeTe and FeSe
The iron chalcogenides FeTe and FeSe belong to the family of iron-based superconductors. We study the magnetism in these compounds in the normal state using the ab initio downfolding scheme developed for strongly correlated electron systems. In deriving ab initio low-energy effective models, we employ the constrained GW method to eliminate the double counting of electron correlations originating from the exchange correlations already taken into account in the density functional theory. By solving the derived ab initio effective models, we reveal that the elimination of the double counting is important in reproducing the bicollinear antiferromagnetic order in FeTe, as is observed in experiments. We also show that the elimination of the double counting induces a unique degeneracy of several magnetic orders in FeSe, which may explain the absence of the magnetic ordering. We discuss the relationship between the degeneracy and the recently found puzzling phenomena in FeSe as well as the magnetic ordering found under pressure. (author)
Gonzalez, Luis E.; Gonzalez, David J
2006-01-01
We have performed orbital free ab initio molecular dynamics simulations in order to study the thermal behaviour of two open surfaces of solid metallic systems, namely the (110) face of fcc Al and the (10-10) face of hcp Mg. Our results reproduce qualitatively both the experimental measurements and previous ab initio calculations performed with the more costly Kohn-Sham approach of Density Functional Theory. These calculations can be viewed as a validation test of the orbital free method for s...
Ab initio studies of complexation of anions to neutral species
Johansson, Patrik [Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden)]. E-mail: patrikj@fy.chalmers.se; Jacobsson, Per [Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goeteborg (Sweden)
2005-06-30
The complexation of simple anions (F{sup -} and Cl{sup -}) to different neutral species, anion-coordinating agents, has been studied using electronic structure calculations. The obtained changes in the equilibrium constants for salt dissolution reactions in different typical electrolyte systems are reported. In addition the lithium ion affinities of the obtained anionic complexes have been calculated. Using the present results we discuss strategies for future usage of anion complexing agents and make recommendations of salt and agent combinations for better lithium battery electrolyte performance.
Ab initio molecular dynamics study of liquid methanol
Handgraaf, J W; Meijer, E J; Handgraaf, Jan-Willem; Erp, Titus S. van; Meijer, Evert Jan
2003-01-01
We present a density-functional theory based molecular-dynamics study of the structural, dynamical, and electronic properties of liquid methanol under ambient conditions. The calculated radial distribution functions involving the oxygen and hydroxyl hydrogen show a pronounced hydrogen bonding and compare well with recent neutron diffraction data, except for an underestimate of the oxygen-oxygen correlation. We observe that, in line with infrared spectroscopic data, the hydroxyl stretching mode is significantly red-shifted in the liquid. A substantial enhancement of the dipole moment is accompanied by significant fluctuations due to thermal motion. Our results provide valuable data for improvement of empirical potentials.
Silicene on metal and metallized surfaces: ab initio studies
The deposition of silicene on several metals is investigated. For fcc crystals the (111) surfaces while for hexagonal ones the (0001) surfaces are used. The Ca(111)1 × 1 substrate is found to be the most promising candidate. The silicene adsorption on Ca-functionalized Si(111)1 × 1 and 2 × 1 surfaces is also studied. The 1 × 1 substrates lead to overlayer silicene with hexagonal symmetry and Dirac cones. However, the Dirac points are below the Fermi level, and small energy gaps are opened. In the case of 2 × 1 surfaces, strong lattice relaxation occurs. Only rudiments of conical linear bands remain visible. (paper)
Ab initio studies of isolated hydrogen vacancies in graphane
Mapasha, R. E.; Molepo, M. P.; Chetty, N.
2016-05-01
We present a density functional study of various hydrogen vacancies located on a single hexagonal ring of graphane (fully hydrogenated graphene) considering the effects of charge states and the position of the Fermi level. We find that uncharged vacancies that lead to a carbon sublattice balance are energetically favorable and are wide band gap systems just like pristine graphane. Vacancies that do create a sublattice imbalance introduce spin polarized states into the band gap, and exhibit a half-metallic behavior with a magnetic moment of 1.00 μB per vacancy. The results show the possibility of using vacancies in graphane for novel spin-based applications. When charging such vacancy configurations, the deep donor (+1/0) and deep acceptor (0/-1) transition levels within the band gap are noted. We also note a half-metallic to metallic transition and a significant reduction of the induced magnetic moment due to both negative and positive charge doping.
Hydrogen adsorption on boron doped graphene: an {\\it ab initio} study
Miwa, R. H.; Martins, T B; Fazzio, A.
2007-01-01
The electronic and structural properties of (i) boron doped graphene sheets, and (ii) the chemisorption processes of hydrogen adatoms on the boron doped graphene sheets have been examined by {\\it ab initio} total energy calculations.
Wolf, T. J. A.; Kuhlman, Thomas Scheby; Schalk, O.;
2014-01-01
Time-resolved photoelectron spectroscopy and ab initio multiple spawning were applied to the ultrafast non-adiabatic dynamics of hexamethylcyclopentadiene. The high level of agreement between experiment and theory associates wavepacket motion with a distinct degree of freedom....
Ab-initio study of thermal expansion in pure graphene
Mann, Sarita; Rani, Pooja; Kumar, Ranjan; Jindal, V. K.
2016-05-01
Graphene is a zero band gap semiconductor with exceptionally high thermal conductivity. The electronic properties having been studied, therole of phonon in contributing to thermal expansion, thermal conductivity and other thermodynamic properties, is required to be investigated. This paper focuses more on thermal expansion. Some others results like phonon dispersion, Grüneisenparameters and bulk modulus,which are essential to estimation of thermal expansion, are also presented. The dynamical matrix was calculated using VASP code using both DFT and DFPTand the phonon frequencies were calculated using phonopy code under harmonic approximation. The linear thermal expansion coefficient of graphene is found to be strongly dependent on temperature but remains negative upto 470 K and positive thereafter, with a room temperature value of -1.44×10-6. The negative expansion coefficient is very interesting and is found to be in conformity with experimental as well as with recent theoretical estimates. There is only qualitative agreement of our results with experimental data and motivates further investigation, primarily on the high negative values of Grüneisen parameters.
Ab initio study of the buckling on silicene and germanene
Martinez-Guerra, Edgar; Hernández, Karla; Cifuentes-Quintal, Eduardo; de Coss, Romeo
2013-03-01
Recently, a new graphene-like silicon structure was discovered: silicene. Since its discovery, silicene has been more exciting than graphene because this is a semiconductor and it should be compatible with silicon-based electronic. Silicon and germanium atoms have similar electronic configurations as those of carbon and this the reason that the bandstructure of silicene and germacene exhibits the Dirac cones at K point, with a very similar linear dispersion around it, like in graphene. The disvintage is that sp2 bonded Si is much less stable than for carbon resulting that to be stable in the planar layer their atoms must buckle. In this work, we calculated the sp character on silicene and germacene to correlate its hibridization with the velocity of electrons and holes at Dirac cones. The calculations were performed using the pseudopotential LCAO method with GGA for the exchange-correlation energy functional. The buckling of silicene and germacene layer was 0.50 and 0.69 Å, respectively. In addition, the sp- character of silicene and germacene buckled was 2.33 and 2.64, respectively. Thus, a detailed analysis on the electronic band structure of these system show that as sp character goes from sp2 to sp3 it is correlated with a decrease of velocity of electrons and holes at Dirac cones. This study is primarly important and it could address a new future to modulate carrier velocities on bidimensional systems. This research was supported by Conacyt under Grant No. 133022.
Ab initio study of boron nitride lines on graphene
Mata-Carrizal, Berenice; Sanginés-Mendoza, Raúl; Martinez, Edgar
2013-03-01
Graphene has unusual electronic properties which make it a promising material for electronic devices. Neverthless, the absence of a band gap sets limitations on its practical applications. Thus, it is crucial to find methods to create and tune the band gap of systems based on graphene. In this way, we explore the modulation of the electronic properties of graphene through doping with boron nitride lines. In particular, we studied the electronic structure of graphene sheets doped with boron nitride lines armchair and zigzag type. The calculations were performed using the pseudopotential LCAO method with a Generalized Gradient Approximation (GGA) for the exchange-correlation energy functional. We found that both doping lines type induce a bandgap and that the energy gap increases as the length of doping lines increases. Accordingly to our DFT calculations, we found that the energy gap on graphene doped with armchair and zigzag lines is due to a two different mechanisms to drain charge from pi- to sigma- orbitals. Thus, we found that doping graphene with boron nitride lines is a useful way to induce and modulate the bandgap on graphene. This research was supported by Consejo Nacional de Ciencia y Tecnología (Conacyt) under Grant No. 133022.
Ab initio study of tungsten defects near the surface
Guerrero, C. L.; Gordillo, N.; Iglesias, R.; Perlado, J. M.; Gonzalez, C.
2016-05-01
A first principles analysis of the behaviour of point defects, namely, self-interstitial atoms, a single vacancy and light impurity atoms such as H and He in tungsten is reported. These defects can be produced in the first wall of the future nuclear fusion reactors due to the high radiation fluxes present. The evolution of defects that appear in the bulk and end up reaching the surface has been followed. An energetic study has been combined with a detailed charge density analysis of the system by means of the SIESTA code. The resulting data have been validated by confronting them with those obtained with a more precise plane wave code, namely VASP. Meanwhile, the structural and the mechanical properties of the system have been positively compared with experimental measurements. Such comparisons have led us to present a new SIESTA basis for tungsten. This complete analysis establishes a nanoscopic view of the phenomena involving the presence of light atoms at native defects in tungsten, paying special attention to the vicinity of surfaces.
Multifunctional properties of zinc oxide: An ab-initio study
Gopal, Priya
In this dissertation, we present our results of first-principles calculations of the electronic and magnetic properties of a range of ZnO-based systems. ZnO is a II-VI wide band gap semiconductor, is a strong piezoelectric, is transparent and can be easily fabricated in a variety of microstructurures from bulk single crystals to thin films to nanostructures. Recent reports also suggest the possibility of magnetic ZnO as a potential spintronic material. Due to its range of properties, ZnO has numerous applications including varistors, piezoelectric transducers, sonars, acoustic devices, solar cells, UV blocking, chemical catalysts and transparent conducting oxides. In this work, we explore the following "multifunctional aspects" of ZnO-based systems. First we calculate the structural, polarization and piezoelectric properties of wurtzite-structure ZnO, MgO and CdO (used in band gap engineering of ZnO) to explore the relationship between optical and piezoelectric functionalities. Second, we investigate wurtzite-structure MnO to understand coexisting magnetic and piezoelectric functionalities. Finally, we study the nature of magnetic interactions in ZnO doped with 3d transition metals to explore the simultaneous magnetic, semiconducting and piezoelectric properties. Our main findings are: (i) Polarization differences between MgO and ZnO, and CdO and ZnO are large suggesting that strong polarization gradients will accompany the changes in band gap induced by Cd or Mg doping. (ii) wurtzite-structure MnO is antiferromagnetic, with a strong magnetostructural coupling and piezoelectric response; this indicates the possibility of coexistence of magnetism and piezoelectricity. (iii) In transition-metal doped ZnO we do not observe a room-temperature ferromagnetic interaction. We find that the magnetic ground state is highly sensitive to details of the calculations such as k-point mesh, details of the geometry optimizations and the choice of exchange correlation functional.
Ab initio Study of Naptho-Homologated DNA Bases
Sumpter, Bobby G [ORNL; Vazquez-Mayagoitia, Alvaro [ORNL; Huertas, Oscar [Universitat de Barcelona; Fuentes-Cabrera, Miguel A [ORNL; Orozco, Modesto [Institut de Recerca Biomedica, Parc Cientific de Barcelona, Barcelona, Spain; Luque, Javier [Universitat de Barcelona
2008-01-01
Naptho-homologated DNA bases have been recently used to build a new type of size expanded DNA known as yyDNA. We have used theoretical techniques to investigate the structure, tautomeric preferences, base-pairing ability, stacking interactions, and HOMO-LUMO gaps of the naptho-bases. The structure of these bases is found to be similar to that of the benzo-fused predecessors (y-bases) with respect to the planarity of the aromatic rings and amino groups. Tautomeric studies reveal that the canonical-like form of naptho-thymine (yyT) and naptho-adenine (yyA) are the most stable tautomers, leading to hydrogen-bonded dimers with the corresponding natural nucleobases that mimic the Watson-Crick pairing. However, the canonical-like species of naptho-guanine (yyG) and naptho-cytosine (yyC) are not the most stable tautomers, and the most favorable hydrogen-bonded dimers involve wobble-like pairings. The expanded size of the naphto-bases leads to stacking interactions notably larger than those found for the natural bases, and they should presumably play a dominant contribution in modulating the structure of yyDNA duplexes. Finally, the HOMO-LUMO gap of the naptho-bases is smaller than that of their benzo-base counterparts, indicating that size-expansion of DNA bases is an efficient way of reducing their HOMO-LUMO gap. These results are examined in light of the available experimental evidence reported for yyT and yyC.
An ab initio molecular dynamics study of the roaming mechanism of the H2+HOC+ reaction
Yu, Hua-Gen
2011-08-01
We report here a direct ab initio molecular dynamics study of the p-/o-H2+HOC+ reaction on the basis of the accurate SAC-MP2 potential energy surface. The quasi-classical trajectory method was employed. This work largely focuses on the study of reaction mechanisms. A roaming mechanism was identified for this molecular ion-molecule reaction. The driving forces behind the roaming mechanism were thoroughly investigated by using a trajectory dynamics approach. In addition, the thermal rate coefficients of the H2+HOC+ reaction were calculated in the temperature range [25, 300] K and are in good agreement with experiments.
Pham, Thi Nu; Ono, Shota; Ohno, Kaoru
2016-04-01
Doing ab initio molecular dynamics simulations, we demonstrate a possibility of hydrogenation of carbon monoxide producing methanol step by step. At first, the hydrogen atom reacts with the carbon monoxide molecule at the excited state forming the formyl radical. Formaldehyde was formed after adding one more hydrogen atom to the system. Finally, absorption of two hydrogen atoms to formaldehyde produces methanol molecule. This study is performed by using the all-electron mixed basis approach based on the time dependent density functional theory within the adiabatic local density approximation for an electronic ground-state configuration and the one-shot GW approximation for an electronic excited state configuration.
Ab-initio density functional theory study of a WO3 NH3-sensing mechanism
WO3 bulk and various surfaces are studied by an ab-initio density functional theory technique. The band structures and electronic density states of WO3 bulk are investigated. The surface energies of different WO3 surfaces are compared and then the (002) surface with minimum energy is computed for its NH3 sensing mechanism which explains the results in the experiments. Three adsorption sites are considered. According to the comparisons of the energy and the charge change between before and after adsorption in the optimal adsorption site O1c, the NH3 sensing mechanism is obtained. (nuclear physics)
Ab initio study of the EFG at the N sites in imidazole
We study the nuclear quadrupole interaction at the nitrogen sites in the molecular and crystalline phases of the imidazole compound. We use PAW which is a state-of-the-art method to calculate the electronic structure and electric field gradient at the nucleus in the framework of the density functional theory. The quadrupole frequencies at both imino and amino N sites are in excellent agreement with measurements. This is the first time that the electric field gradient at crystalline imidazole is correctly treated by an ab initio theoretical approach.
Ab initio simulations and neutron scattering studies of structure and dynamics in PdH
The work presented in this PhD thesis is concerned with the interpretation of the neutron scattering measurements from the palladium hydrogen system by means of ab initio electronic structure calculations. The motivation of performing such calculations was due to recent neutron scattering studies on this system that showed a strong directional dependence to the dynamical structure factor together with a complex dependence on energy. Here we attempt to describe the origin of these features by ab initio simulations of the dynamical structure factor. The method assumes an adiabatic separation of the motion of the proton and palladium atoms. The proton wave functions are calculated by a direct solution of the associated single-particle Schroedinger equation using a plane wave basis set method and a mapping of the adiabatic surface. The Fourier components of the adiabatic potential are obtained from LDA pseudopotential calculations. Using Fermi's golden rule within the Born approximation we were then able to calculate the dynamical structure factor, S(Q,ω), for exciting the proton from its ground state to various excited states as a function of the magnitude and direction of the scattering wave vector. The results are in agreement with the inelastic neutron scattering spectra and allow us to identify the origin of previous inexplicable features, in particular the strong directional dependence to the experimental data. The method was extended to investigate the expansion of the equilibrium lattice constant as a function of the H isotope when the zero-point energy of the proton/deuterium is explicitly taken into account in the relaxation process. The results we obtained predicted a bigger lattice constant for the hydride, as expected. Furthermore, other complex ab initio calculations were carried out in order to describe the origin of the large optic dispersion, seen previously in the coherent neutron scattering data. Our calculated dispersion proved to be in good
Ab initio, density functional theory and structural studies of 4-amino-2-methylquinoline
Arjunan, V.; Saravanan, I.; Ravindran, P.; Mohan, S.
2009-10-01
The Fourier transform infrared (FTIR) and FT-Raman spectra of 4-amino-2-methylquinoline (AMQ) have been recorded in the range 4000-400 and 4000-100 cm -1, respectively. The experimental vibrational frequency was compared with the wavenumbers obtained theoretically by ab initio HF and DFT-B3LYP gradient calculations employing the standard 6-31G** and high level 6-311++G** basis sets for optimised geometry of the compound. The complete vibrational assignment and analysis of the fundamental modes of the compounds were carried out using the experimental FTIR and FT-Raman data, and quantum mechanical studies. The geometry and normal modes of vibration obtained from the HF and DFT methods are in good agreement with the experimental data. The potential energy distribution of the fundamental modes was calculated with ab initio force fields utilising Wilson's FG matrix method. The NH -π interactions and the influence of amino and methyl groups on the skeletal modes are investigated.
Ab Initio Study of Hot Carriers in the First Picosecond after Sunlight Absorption in Silicon
Bernardi, Marco; Vigil-Fowler, Derek; Lischner, Johannes; Neaton, Jeffrey B.; Louie, Steven G.
2014-06-01
Hot carrier thermalization is a major source of efficiency loss in solar cells. Because of the subpicosecond time scale and complex physics involved, a microscopic characterization of hot carriers is challenging even for the simplest materials. We develop and apply an ab initio approach based on density functional theory and many-body perturbation theory to investigate hot carriers in semiconductors. Our calculations include electron-electron and electron-phonon interactions, and require no experimental input other than the structure of the material. We apply our approach to study the relaxation time and mean free path of hot carriers in Si, and map the band and k dependence of these quantities. We demonstrate that a hot carrier distribution characteristic of Si under solar illumination thermalizes within 350 fs, in excellent agreement with pump-probe experiments. Our work sheds light on the subpicosecond time scale after sunlight absorption in Si, and constitutes a first step towards ab initio quantification of hot carrier dynamics in materials.
Ab initio study of Ni2MnGa under shear deformation
Zelený Martin
2015-01-01
Full Text Available The effect of shear deformation on Ni2MnGa magnetic shape memory alloy has been investigated using ab initio electronic structure calculations. We used the projector-augmented wave method for the calculations of total energies and stresses as functions of applied affine shear deformation. The studied nonmodulated martensite (NM phase exhibits a tetragonally distorted L21 structure with c/a > 1. A large strain corresponding to simple shears in {001}, {100} and {100} systems was applied to describe a full path between two equivalent NM lattices. We also studied {101} shear which is related to twining of NM phase. Twin reorientation in this system is possible, because applied positive shear results in path with significantly smaller energetic barrier than for negative shear and for shears in other studied systems. When the full relaxation of lattice parameters is allowed, the barriers further strongly decrease and the structures along the twinning path can be considered as orthorhombic.
Comparative studies of the spectroscopy of CuCl2: DFT versus standard ab initio approaches.
Ramírez-Solís, A; Poteau, R; Vela, A; Daudey, J P
2005-04-22
The X2Pi g-2Sigma g+, X2Pi g-2Delta g, X2Pi g-2Sigma u+, X2Pi g-2Pi u transitions on CuCl2 have been studied using several exchange-correlation functionals from the various types of density functional theory (DFT) approaches like local density approximation (LDA), generalized gradient approximation (GGA), hybrid and meta-GGA. The results are compared with the experience and with those coming from the most sophisticated nondynamic and dynamic electronic correlation treatments using the same relativistic effective core potentials and especially developed basis sets to study the electronic structure of the five lowest states and the corresponding vertical and adiabatic transition energies. The calculated transition energies for three of the hybrid functionals (B3LYP, B97-2, and PBE0) are in very good agreement with the benchmark ab initio results and experimental figures. All of the other functionals largely overestimate the X2Pi g-2Sigma g+ and X2Pi g-2Delta g transition energies, many of them even placing the 2Delta g ligand field state above the charge transfer 2Pi u and 2Sigma u+ states. The relative weight of the Hartree-Fock exchange in the definition of the functional used appears to play a key role in the accurate description of the LambdaSSigma density defined by the orientation of the 3d hole (sigma, pi, or delta) on Cu in the field of both chlorine atoms, but no simple connection of this weight with the quality of the spectra has been found. Mulliken charges and spin densities are carefully analyzed; a possible link between the extent of spin density on the metal for the X2Pi g state and the performance of the various functionals was observed, suggesting that those that lead to the largest values (close to 0.65) are the ones that best reproduce these four transitions. Most functionals lead to a remarkably low ionicity for the three ligand field states even for the best performing functionals, compared to the complete active space (SCF) (21, 14) ab initio
Tunneling of electrons via rotor-stator molecular interfaces: combined ab initio and model study
Petreska, Irina; Pejov, Ljupco; Kocarev, Ljupco
2015-01-01
Tunneling of electrons through rotor-stator anthracene aldehyde molecular interfaces is studied with a combined ab initio and model approach. Molecular electronic structure calculated from first principles is utilized to model different shapes of tunneling barriers. Together with a rectangular barrier, we also consider a sinusoidal shape that captures the effects of the molecular internal structure more realistically. Quasiclassical approach with the Simmons' formula for current density is implemented. Special attention is paid on conformational dependence of the tunneling current. Our results confirm that the presence of the side aldehyde group enhances the interesting electronic properties of the pure anthracene molecule, making it a bistable system with geometry dependent transport properties. We also investigate the transition voltage and we show that confirmation dependent field emission could be observed in these molecular interfaces at realistically low voltages. The present study accompanies our previ...
Ab-initio study of magnetism behavior in TiO2 semiconductor with structural defects
Zarhri, Z.; Houmad, M.; Ziat, Y.; El Rhazouani, O.; Slassi, A.; Benyoussef, A.; El Kenz, A.
2016-05-01
Magnetic, electronic and structural properties of titanium dioxide material with different structural defects are studied using the first-principles ab-initio calculations and the Korringa-Kohn-Rostoker method (KKR) combined with the coherent potential approximation (CPA) method in connection with the local density approximation (LDA). We investigated all structural defects in rutile TiO2 such as Titanium interstitial (Tii), Titanium anti-sites (Tio), Titanium vacancies (VTi), Oxygen interstitial (Oi), Oxygen anti-sites (OTi) and oxygen vacancies (Vo). Mechanisms of hybridization and interaction between magnetic atoms are investigated. The transition temperature is computed using the Mean Field Approximation (MFA).Magnetic stability energy of ferromagnetic and disordered local moment states is calculated to determine the most stable state. Titanium anti-sites have a half-metallic aspect. We also studied the change type caused by structural defects in this material.
Hydrogen-Water Mixtures in Giant Planet Interiors Studied with Ab Initio Simulations
Soubiran, Francois
2015-01-01
We study water-hydrogen mixtures under planetary interior conditions using ab initio molecular dynamics simulations. We determine the thermodynamic properties of various water-hydrogen mixing ratios at temperatures of 2000 and 6000 K for pressures of a few tens of GPa. These conditions are relevant for ice giant planets and for the outer envelope of the gas giants. We find that at 2000 K the mixture is in a molecular regime, while at 6000 K the dissociation of hydrogen and water is important and affects the thermodynamic properties. We study the structure of the liquid and analyze the radial distribution function. We provide estimates for the transport properties, diffusion and viscosity, based on autocorrelation functions. We obtained viscosity estimates of the order of a few tenths of mPa.s for the conditions under consideration. These results are relevant for dynamo simulations of ice giant planets.
Atomic carbon chains as spin-transmitters: An ab initio transport study
Fürst, Joachim Alexander; Brandbyge, Mads; Jauho, Antti-Pekka
2010-01-01
An atomic carbon chain joining two graphene flakes was recently realized in a ground-breaking experiment by Jin et al. (Phys. Rev. Lett., 102 (2009) 205501). We present ab initio results for the electron transport properties of such chains and demonstrate complete spin-polarization of the transmi...
Ab initio study of long-period superstructures in close-packed A3B compounds
Rosengaard, N. M.; Skriver, Hans Lomholt
1994-01-01
We have performed ab initio calculations of the stability of one-dimensional long-period superstructures in Cu3Pd, Cu3Al, and Ag3Mg by means of an interface Green's function technique based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations. The...
Raman and ab initio studies of simple and binary 1-alkyl-3-methylimidazolium ionic liquids
Berg, R.W.; Deetlefs, M.; Seddon, K.R.;
2005-01-01
Raman spectra of the ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF6]), 1-hexyl-3-methylimidazolium chloride ([C(6)mim]Cl), and 1-hexyl-3-methylimidazolium hexafluorophosphate ([C(6)mim][PF6]), and binary mixtures thereof, have been assigned using ab initio MP2...
Study on the surface hydroxyl group on solid breeding materials by ab-initio calculations
Tanaka, Satoru; Taniguchi, Masaki [Tokyo Univ. (Japan). Faculty of Engineering
1996-10-01
The nature of -OH on the surface of Li{sub 2}O was analyzed with the ab-initio quantum chemical calculation technique. Calculation results showed that the stretching vibration of O-H is affected by the chemical species around the -OH. (author)
Structural and electronic properties of lead nanowires: Ab-initio study
Highlights: → In the present revised manuscript entitled 'Structural and Electronic Properties of Lead Nanowires: Ab-initio study', we have analyzed the stability, electronic properties as well as ground state properties of various atomic configurations of Lead nanowires. → The two-atom zigzag shaped lead nanowire with highest binding energy and lowest total energy has been confirmed as the most stable structure out of the six atomic configurations taken into consideration. → The electronic band structure and density of states have been described in detail with a remarkable observation in case of three-atom triangular lead nanowire having a very small band gap while other atomic configurations are found to be metallic. → The bulk modulus and pressure derivatives for all the stable geometries have also been computed and discussed in the manuscript. The mechanical strength of nanowires has also been discussed in terms of its bulk modulus. → The two-atom ladder shaped nanowire with highest bulk modulus, defends this structure as mechanically stronger than the other tested structure. - Abstract: Ab-initio self-consistent study has been performed to analyze the stability of lead nanowires in its six stable configurations like linear, zigzag, triangular, ladder, square and dumbbell. In the present study, the lowest energy structures have been analyzed under the revised Perdew-Burke-Ernzerhof (revPBE) parameterization of generalized gradient approximation (GGA) potential. The two-atom zigzag shaped atomic configuration with highest binding energy and lowest total energy has been confirmed as the most stable structure out of the six atomic configurations. The electronic band structure and density of states have been discussed in detail with a remarkable observation in case of three-atom triangular lead nanowire having a very small band gap while other configurations are found to be metallic. Bulk modulus, pressure derivatives and lattice parameters for different
Ab Initio Study of BiFeO3: Thermodynamic Stability Conditions.
Heifets, E; Kotomin, E A; Bagaturyants, A A; Maier, J
2015-07-16
BiFeO3 is investigated intensively, mainly as a multiferroic material. In this paper, the state-of-the-art ab initio hybrid functional approach with atomic basis sets was employed for a study of the stability range of BiFeO3 with respect to its decomposition into binary oxides and elementary metals, as a function of temperature and oxygen partial pressure. The calculated atomic and electronic structure of BiFeO3 was compared with previous LDA+U calculations using plane-wave basis sets. Based on performed calculations, the phase diagram was constructed, which allows us to predict the stability region of stoichiometric BiFeO3. PMID:26266871
Ab Initio Studies on Hematite Surface and the Adsorption of Phosphate
Swati Chaudhury
2014-01-01
Full Text Available This investigation explores the ab initio DFT method for understanding surface structure of hematite and the nature and energetics of phosphate adsorption. Using the full potential linearized plane wave method (FP-LAPW, we derived the structure and energies of various magnetic forms of hematite. The antiferromagnetic (AFM form was observed to be the most stable. Hematite surfaces with Fe-termination, O-termination, or OH-termination were studied. The OH-terminated surface was the most stable. Stability of hematite surfaces follows the order OH-termination > Fe-termination > O-termination. Thus, surface reaction with hematite would occur with the OH at the surface and not with Fe atoms. The structure of phosphate adsorbed on hematite was derived. Bonding is through the H atom of the OH at the surface. An alternative mechanism of phosphate adsorption on hematite has been derived. Adsorption energy is high and suggests chemisorption rather than physisorption of phosphate on hematite.
An ab initio cluster model study of the magnetic coupling in KNiF3
Casanovas, Jordi; Illas, Francesc
1994-06-01
Cluster models of increasing complexity have been used to model magnetic interactions in KNiF3. These clusters contain two or four magnetic centers plus the bridge F- anions and different representations of the remaining of the crystal. The magnetic coupling constant has been obtained by computing ab initio wave functions for different spin states. These wave functions explicitly include internal and external correlation effects. Several sets of Gaussian functions have been tested and many sets of molecular orbitals have been considered in order to study the physical origin of magnetism in KNiF3. The calculated magnetic coupling constant differs from model to model but shows a fairly good convergence to the experimental result. The use of different cluster models permits to separate the magnetic coupling constant in several contributions. These are the delocalization of magnetic orbitals, the external correlation, and the collective effects normally hidden in the two body operator of the Heisenberg Hamiltonian.
Ab initio study of one-dimensional disorder on III-V semiconductor surfaces
Romanyuk, O. [Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, 162 00 Prague 6 (Czech Republic); Grosse, F.; Braun, W. [Paul-Drude-Institut fuer Festkoerperelektronik, Hausvogteiplatz 5-7, 10117 Berlin (Germany)
2010-02-15
Atomic disorder on GaSb(001) and GaAs(001) surfaces is studied by ab initio calculations within density functional theory (DFT). Surface energies are computed for GaSb(001) {beta} (4 x 3), {beta} (4 x 3)A{sub 1}, (4 x 6), and GaAs(001) {beta} 2(2 x 4), c (4 x 4) reconstructions. The computed energy differences do not exceed 1 meV / (1 x 1) which is in agreement with observed one-dimensional disorder on GaSb(001) and GaAs(001) surfaces at elevated temperatures for {beta} and {beta} 2 stoichiometries, respectively. Deviations in bond lengths due to disorder with respect to the ordered ground state phases are calculated (copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
An ab initio study of three (ethane-1,2 diol/water) complexes
Manivet, Philippe; Masella, Michel
1998-05-01
Three (ethane-1,2 diol/water) complexes have been studied using ab initio calculations at the MP2 level. In two complexes, the ethane-1,2 diol structure is close to its gas phase experimental structure (presence of an intramolecular hydrogen bond HB and the O-C-C-O dihedral angle is gauche) while the intramolecular HB is disrupted by the presence of a water molecule in the third ( tGg'a). Computations have shown that most of the experimental observations regarding the solvation of ethane-1,2 diol in water may be reproduced only by considering the tGg'a complex (absence of intramolecular HB, O-C-C-O dihedral angle of 72-74°), which is also more stable than the other two by 2 kcal mol -1.
Structural properties of iron nitride on Cu(100): An ab-initio molecular dynamics study
Heryadi, Dodi
2011-01-01
Due to their potential applications in magnetic storage devices, iron nitrides have been a subject of numerous experimental and theoretical investigations. Thin films of iron nitride have been successfully grown on different substrates. To study the structural properties of a single monolayer film of FeN we have performed an ab-initio molecular dynamics simulation of its formation on a Cu(100) substrate. The iron nitride layer formed in our simulation shows a p4gm(2x2) reconstructed surface, in agreement with experimental results. In addition to its structural properties, we are also able to determine the magnetization of this thin film. Our results show that one monolayer of iron nitride on Cu(100) is ferromagnetic with a magnetic moment of 1.67 μ B. © 2011 Materials Research Society.
Femtosecond Laser Processing of Germanium: An Ab Initio Molecular Dynamics Study
Ji, Pengfei
2016-01-01
An ab initio molecular dynamics study of femtosecond laser processing of germanium is presented in this paper. The method based on the finite temperature density functional theory is adopted to probe the structural change, thermal motion of the atoms, dynamic property of the velocity autocorrelation, and the vibrational density of states. Starting from a cubic system at room temperature (300 K) containing 64 germanium atoms with an ordered arrangement of 1.132 nm in each dimension, the femtosecond laser processing is simulated by imposing the Nose Hoover thermostat to the electronic subsystem lasting for ~100 fs and continuing with microcanonical ensemble simulation of ~200 fs. The simulation results show solid, liquid and gas phases of germanium under adjusted intensities of the femtosecond laser irradiation. We find the irradiated germanium distinguishes from the usual germanium crystal by analyzing their melting and dynamic properties.
High-pressure study of ScVO4 by Raman scattering and ab initio calculations
Panchal, V.; Manjón, F. J.; Errandonea, D.; Rodriguez-Hernandez, P.; López-Solano, J.; Muñoz, A.; Achary, S. N.; Tyagi, A. K.
2011-02-01
We report results of experimental and theoretical lattice-dynamics studies on scandium orthovanadate up to 35 GPa. Raman-active modes of the low-pressure zircon phase are measured up to 8.2 GPa, where the onset of an irreversible zircon-to-scheelite phase transition is detected. Raman-active modes in the scheelite structure are observed up to 16.5 GPa. Beyond 18.2 GPa we detected a gradual splitting of the Eg modes of the scheelite phase, indicating the onset of a second phase transition. Raman symmetries, frequencies, and pressure coefficients in the three phases of ScVO4 are discussed in the light of ab initio lattice-dynamics calculations that support the experimental results. The results on all the three phases of ScVO4 are compared with those previously reported for related orthovanadates.
Study of the atomic rare gas behaviour by ab initio calculations
The atomic behaviour of helium and krypton in uranium dioxide has been studied using an ab initio simulation technique. Incorporation energies and solution energies of these two rare gases have been calculated. Krypton atoms are found to be insoluble in this nuclear fuel whatever the trap considered and their presence in the lattice induces swelling when they are located in interstitial sites or in oxygen vacancies. Due to its small atomic size, the predicted helium behaviour is very different. Indeed, helium is found to be soluble in stoichiometric and hyper-stoichiometric uranium dioxide in the presence of uranium vacancies or divacancies constituted by one uranium and one oxygen vacancy. Moreover helium atoms induce a lattice parameter contraction except in interstitial sites where a slight expansion is calculated. (author)
High multiplicity states in disordered carbon systems: Ab initio and semiempirical study
Stability of non-zero spin projection states for disordered carbon clusters of low symmetry were examined using semiempirical and ab initio methods. The study proves previous results of V.D. Khavryuchenko, Y.A. Tarasenko, V.V. Strelko, O.V. Khavryuchenko, V.V. Lisnyak, Int. J. Mod. Phys. B 21 (2007) 4507, obtained for the large polyaromatic hydrocarbons clusters and shows that the phenomenon is intrinsic for carbon-rich systems and independent of their symmetries. The electronic properties of the carbon clusters may alter from insulating to semiconducting upon change of C/H ratio and stabilization of non-zero spin projection states. A partial collectivization of the electrons is observed in deeply carbonized carbon clusters in higher Sz states.
Tao, Kun; Stepanyuk, V.S.; Bruno, P.;
2008-01-01
The state of the art ab initio calculations reveal the effect of a scanning tunneling microscopy tip on magnetic properties and conductance of a benzene-adatom sandwich on Cu(001). We concentrate on a benzene-Co system interacting with a Cr tip. Our studies give a clear evidence that magnetism an...
A Photoelectron Spectroscopy and ab initio Study of B3- and B4- Anions and Their Neutrals
Zhai, Hua-Jin; Wang, Lai S.; Alexandrova, A N.; Boldyrev, Alexander I.; Zakrzewski, V G.
2003-11-06
The two smallest boron clusters (B3 and B4) in their neutral and anionic forms were studied by photoelectron spectroscopy and ab initio calculations. Vibrationally resolved photoelectron spectra were observed for B3- at three photon energies (355, 266, and 193 nm) and the electron affinity of B3 was measured to be+0.02 eV.
Ab-initio study of magnetic properties and phase transitions in Ga (Mn) N with Monte Carlo approach
Sbai, Y.; Ait Raiss, A.; Salmani, E. [LMPHE (URAC 12), Faculty of Science, Mohammed V University, Av. Ibn Batouta, Rabat (Morocco); Bahmad, L., E-mail: Bahmad@fsr.ac.ma [LMPHE (URAC 12), Faculty of Science, Mohammed V University, Av. Ibn Batouta, Rabat (Morocco); Benyoussef, A. [LMPHE (URAC 12), Faculty of Science, Mohammed V University, Av. Ibn Batouta, Rabat (Morocco); Institute of Nanomaterials and Nanotechnologies, MAScIR, Rabat (Morocco); Hassan II Academy of Science and Technology, Rabat (Morocco)
2015-12-15
On the basis of ab-initio calculations and Monte Carlo simulations the magnetic and electronic properties of Gallium nitride (GaN) doped with the transition metal Manganese (Mn) were studied. The ab initio calculations were done using the AKAI–KKR–CPA method within the Local Density Approximation (LDA) approximation. We doped our Diluted Magnetic Semiconductor (DMS), with different concentrations of magnetic impurities Mn and plotted the density of state (DOS) for each one. Showing a half-metallic behavior and ferromagnetic state especially for Ga{sub 0.95}Mn{sub 0.05}N making this DMS a strong candidate for spintronic applications. Moreover, the magnetization and susceptibility of our system as a function of the temperature has been calculated and give for various system size L to study the size effect. In addition, the transition temperature was deduced from the peak of the susceptibility. The Ab initio results are in good agreement with literature especially for (x=0.05) of Mn which gives the most interesting results. - Highlights: • The AKAI–KKR–CPA method has been applied to study the doped compound GaN:Mn. • The local density approximation (LDA) has been applied. • The ab-initio calculations have been performed. • The density of states (DOS) have been plotted for differents doping concentrations, using Monte Carlo simulations.
Conformational space of clindamycin studied by ab initio and full-atom molecular dynamics.
Kulczycka-Mierzejewska, Katarzyna; Trylska, Joanna; Sadlej, Joanna
2016-01-01
Molecular dynamics (MD) simulations allow determining internal flexibility of molecules at atomic level. Using ab initio Born-Oppenheimer molecular dynamics (BOMD), one can simulate in a reasonable time frame small systems with hundreds of atoms, usually in vacuum. With quantum mechanics/molecular mechanics (QM/MM) or full-atom molecular dynamics (FAMD), the influence of the environment can also be simulated. Here, we compare three types of MD calculations: ab initio BOMD, hybrid QM/MM, and classical FAMD. As a model system, we use a small antibiotic molecule, clindamycin, which is one of the lincosamide antibiotics. Clindamycin acquires two energetically stable forms and we investigated the transition between these two experimentally known conformers. We performed 60-ps BOMD simulations in vacuum, 50-ps QM/MM, and 100-ns FAMD in explicit water. The transition between two antibiotic conformers was observed using both BOMD and FAMD methods but was not noted in the QM/MM simulations. PMID:26733483
Melting of sodium under high pressure. An ab-initio study
González, D. J.; González, L. E. [Departamento de Física Teórica, Atómica y Optica, Universidad de Valladolid, 47011 Valladolid (Spain)
2015-08-17
We report ab-initio molecular dynamics simulations of dense liquid/solid sodium for a pressure range from 0 to 100 GPa. The simulations have been performed with the orbital free ab-initio molecular dynamics method which, by using the electron density as the basic variable, allows to perform simulations with large samples and for long runs. The calculated melting curve shows a maximum at a pressure ≈ 30 GPa and it is followed by a long, steep decrease. These features are in good agreement with the experimental data. For various pressures along the melting curve, we have calculated several liquid static properties (pair distribution functions, static structure factors and short-range order parameters) in order to analyze the structural effects of pressure.
Experimental studies and ab initio calculations on characteristics of the C state of SF2 radical
SF2 radicals were generated by a pulsed dc discharge in the mixture gas beam of SF2 and Ar. The (2+1) resonance-enhanced multiphoton ionization (REMPI) excitation spectroscopy of SF2 radical was obtained between 325 and 365 nm. The SF+ ion signals were also observed in the same wavelength range. The analysis shows that the spectrum can be assigned as the two-photon resonant excitation of SF2 radical (B-tilde1 B1 and (C-tilde1 A1 states). And also, ab initio calculations suggest that the C-tilde state is a bonding state with Rydberg characteristic. The potential energy surfaces (PESs) of SF2 and SF2+ by ab initio calculations suggest that SF+ ions originate from dissociation processes of excited SF2+ ions. (author)
How Is Acetylcholinesterase Phosphonylated by Soman? An Ab Initio QM/MM Molecular Dynamics Study
Sirin, Gulseher Sarah; Zhang, Yingkai
2014-01-01
Acetylcholinesterase (AChE) is a crucial enzyme in the cholinergic nerve system that hydrolyzes acetylcholine (ACh) and terminates synaptic signals by reducing the effective concentration of ACh in the synaptic clefts. Organophosphate compounds irreversibly inhibit AChEs, leading to irreparable damage to nerve cells. By employing Born–Oppenheimer ab initio QM/MM molecular dynamics simulations with umbrella sampling, a state-of-the-art approach to simulate enzyme reactions, we have characteriz...
Ab initio study of one-dimensional disorder on III-V semiconductor surfaces
Romanyuk, Olexandr; Grosse, F.; Braun, W.
2010-01-01
Roč. 7, č. 2 (2010), s. 330-333. ISSN 1862-6351 R&D Projects: GA AV ČR KAN300100802 Grant ostatní: German Resarch Fondation(DE) GZ:436 TSE 113/62/0-1 Institutional research plan: CEZ:AV0Z10100521 Keywords : atomic disorder * ab initio * semiconductor * reconstruction Subject RIV: BM - Solid Matter Physics ; Magnetism
Ab initio study of antiphase boundaries and stacking faults in L12 and DO22 compounds
Rosengaard, N. M.; Skriver, Hans Lomholt
1994-01-01
We have performed ab initio calculations of the energies of antiphase boundaries as well as complex and superlattice intrinsic stacking faults in nine intermetallic compounds observed in the face-centered-cubic L1(2) and DO22 structures. The calculations were performed by means of a Green's funct......'s function technique based on the linear-muffin-tin-orbitals method within the tight-binding and atomic-sphere approximations....
Ab initio study of phase transformations in transition-metal disilicides
Káňa, Tomáš; Šob, Mojmír; Vitek, V.
2011-01-01
Roč. 19, č. 7 (2011), s. 919-926. ISSN 0966-9795 R&D Projects: GA AV ČR IAA100100920; GA MŠk(CZ) OC10008 Institutional research plan: CEZ:AV0Z20410507 Keywords : silicides various * phase transformation * plastic deformation mechanisms * defects * dislocation geometry and arrangement * ab-initio calculations * aero- engine components Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 1.649, year: 2011
Ab initio study of ideal tensile strength and mechanical stability of transition-metal disilicides
Friák, Martin; Šob, Mojmír; Vitek, V.
2003-01-01
Roč. 68, č. 18 (2003), s. 184101-1 - 181101-10. ISSN 0163-1829 R&D Projects: GA AV ČR IAA1041302; GA ČR GA202/03/1351; GA MŠk OC 523.90 Institutional research plan: CEZ:AV0Z2041904 Keywords : ab initio calculations * electronic structure * theoretical tensile strength Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 2.962, year: 2003
Ehala, Sille; Dybal, Jiří; Makrlík, E.; Kašička, Václav
2009-01-01
Roč. 32, č. 4 (2009), s. 597-604. ISSN 1615-9306 R&D Projects: GA ČR(CZ) GA203/06/1044; GA ČR(CZ) GA203/08/1428; GA AV ČR 1ET400500402 Institutional research plan: CEZ:AV0Z40550506; CEZ:AV0Z40500505 Keywords : capillary affinity electrophoresis * valinomycin * ab initio calculation Subject RIV: CB - Analytical Chemistry, Separation Impact factor: 2.551, year: 2009
Hemzalová, P.; Friák, Martin; Šob, Mojmír; Ma, D.; Udyansky, A.; Raabe, D.; Neugebauer, J.
2013-01-01
Roč. 88, č. 17 (2013), Art. no. 174103. ISSN 1098-0121 R&D Projects: GA ČR(CZ) GAP108/12/0311; GA ČR GD106/09/H035; GA AV ČR IAA100100920 Institutional support: RVO:68081723 Keywords : nitrides * ab initio * thermodynamics * elasticity Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.664, year: 2013
Ab initio molecular dynamics study of liquid sodium and cesium up to critical point
Yuryev, Anatoly A. [Institute of Metallurgy of Ural Branch of the Russian Academy of Sciences, Amundsen st. 101,620016, Yekaterinburg (Russian Federation); Ural Federal University, Vira st. 19, 620002, Yekaterinburg (Russian Federation); Gelchinski, Boris R. [Institute of Metallurgy of Ural Branch of the Russian Academy of Sciences, Amundsen st. 101,620016, Yekaterinburg (Russian Federation)
2015-08-17
Ab initio modeling of liquid metals Na and K is carried out using the program SIESTA. We have determined the parameters of the model (the optimal step, the number of particles, the initial state etc) and calculated a wide range of properties: the total energy, pair correlation function, coefficient of self-diffusion, heat capacity, statistics of Voronoi polyhedra, the density of electronic states up to the critical temperature.
Ab initio studies on the lattice thermal conductivity of silicon clathrate frameworks II and VIII
Härkönen, Ville J.; Karttunen, Antti J.
2015-01-01
The lattice thermal conductivities of silicon clathrate frameworks II and VIII are investigated by using ab initio lattice dynamics and iterative solution of the linearized Boltzmann transport equation(BTE) for phonons. Within the temperature range 100-350 K, the clathrate structures II and VIII were found to have lower lattice thermal conductivity values than silicon diamond structure (d-Si) by factors of 1/2 and 1/5, respectively. The main reason for the lower lattice thermal conductivity o...
Ab initio studies of the kinetic isotope effect of the CH4 + OH atmospheric reaction
Lasaga, Antonio C.; Gibbs, G. V.
1991-07-01
High level ab initio calculations have been carried out on the C-13 - C-12 kinetic isotope effect of the CH4 + OH reaction in the atmosphere. The results agree quite well with both the absolute value of the isotope effect and the temperature dependence of the effect, based on new experimental data. The calculated kinetic isotope effect supports a bigger effect of biomass burning on the methane global budget.
Ab InitioStudy of Hot Carriers in the First Picosecond after Sunlight Absorption in Silicon
Bernardi, Marco; Vigil-Fowler, Derek; Lischner, Johannes; Neaton, Jeffrey B.; Louie, Steven G.
2014-01-01
Hot carrier thermalization is a major source of efficiency loss in solar cells. Because of the subpicosecond time scale and complex physics involved, a microscopic characterization of hot carriers is challenging even for the simplest materials. We develop and apply an ab initio approach based on density functional theory and many-body perturbation theory to investigate hot carriers in semiconductors. Our calculations include electron-electron and electron-phonon interactions, and require no e...
Discovering chemistry with an ab initio nanoreactor
Wang, Lee-Ping; Titov, Alexey; McGibbon, Robert; Liu, Fang; Pande, Vijay S.; Martínez, Todd J.
2014-01-01
Chemical understanding is driven by the experimental discovery of new compounds and reactivity, and is supported by theory and computation that provides detailed physical insight. While theoretical and computational studies have generally focused on specific processes or mechanistic hypotheses, recent methodological and computational advances harken the advent of their principal role in discovery. Here we report the development and application of the ab initio nanoreactor – a highly accelerat...
On Possible Pitfalls in ab initio QM/MM Minimization Approaches For Studies of Enzymatic Reactions
Klähn, Marco; Braun-Sand, Sonja; Rosta, Edina; Warshel, Arieh
2006-01-01
Reliable studies of enzymatic reactions by combined quantum mechanics /molecular mechanics (QM/MM) approaches, with an ab initio description of the quantum region, presents a major challenge to computational chemists. The main problem is the need for a very large computer time for the evaluation of the QM energy, which in turn makes it extremely challenging to perform proper configurational sampling. A seemingly reasonable alternative is to perform energy minimization studies of the type used in gas phase ab initio studies. However, it is hard to see why such an approach should give reliable results in protein active sites. In order to examine the problems with energy minimization QM/MM approaches we chose the hypothetical reaction of a metaphosphate ion with water in the Ras•GAP complex. This hypothetical reaction served as a simple benchmark reaction. The possible problems with the QM/MM minimization were explored by generating several protein configurations from long MD simulations and using energy minimization and scanning of the reaction coordinates to evaluate the corresponding potential energy surfaces of the reaction for each of these different protein configurations. Comparing these potential energy surfaces, we found major variations of the minima of the different total potential energy surfaces. Furthermore, the reaction energies and activation energies also varied significantly even for similar protein configurations. The specific coordination of a magnesium ion, present in the active center of the protein complex, turned out to influence the energetics of the reaction in a major way and a direct coordination to the reactant leads to an increase of the activation energy by 17 kcal/mol. This study demonstrates that energy minimizations starting from a single protein structure could lead to major errors in calculations of activation free energies and binding free energies. Thus we believe that extensive samplings of the configurational space of the
Kühne, Thomas D
2012-01-01
Computer simulations and molecular dynamics in particular, is a very powerful method to provide detailed and essentially exact informations of classical many-body problems. With the advent of \\textit{ab-initio} molecular dynamics, where the forces are computed on-the-fly by accurate electronic structure calculations, the scope of either method has been greatly extended. This new approach, which unifies Newton's and Schr\\"odinger's equations, allows for complex simulations without relying on any adjustable parameter. This review is intended to outline the basic principles as well as a survey of the field. Beginning with the derivation of Born-Oppenheimer molecular dynamics, the Car-Parrinello method as well as novel hybrid scheme that unifies best of either approach are discussed. The predictive power is demonstrated by a series of applications ranging from insulators to semiconductors and even metals in condensed phases.
The crystallization process of liquid vanadium studied by ab initio molecular dynamics
We present a study of the crystallization process in liquid vanadium over a temperature range from 3000 K down to 1500 K using ab initio molecular dynamics simulations. Short-range order evolution during solidification is studied using various structural analysis methods. We show that the icosahedral-like short-range order is detected in the stable liquid phase and grows upon supercooling. The system undergoes a first-order phase transition (from a liquid to a solid state) at a temperature of about 1600 K. The crystal nucleation process is further studied using the time–temperature transformation mechanism by annealing the system at 1650 K. The nucleation is examined using bond-orientational order and density fluctuation analysis. Our finding is that various precursors appear in the region of high bond-orientational order with the majority having body-centered cubic (bcc)-like symmetry. This bcc-like region grows on annealing via thermal fluctuations. Our results reveal that the bond-orientational order precedes the density fluctuation, and is the main driving factor for nucleation. (papers)
Ab initio study of basal slip in Nb2AlC
Using ab initio calculations, we have studied shearing in Nb2AlC, where NbC and Al layers are interleaved. The stress-strain analysis of this deformation mode reveals Nb-Al bond breaking, while the Nb-C bond length decreases by 4.1%. Furthermore, there is no evidence for phase transformation during deformation. This is consistent with basal slip and may be understood on the basis of the electronic structure: bands below the Fermi level are responsible for the dd bonding between NbC basal planes and only a single band with a weak dd interaction is not resistant to shearing, while all other bands are unaffected. The Al-Nb bonding character can be described as mainly metallic with weak covalent-ionic contributions. Our study demonstrates that Al layers move with relative ease under shear strain. Phase conservation upon shearing is unusual for carbides and may be due to the layered nature of the phase studied. Here, we describe the electronic origin of basal slip in Nb2AlC, the atomic mechanism which enables reversible plasticity in this class of materials
Tunneling of electrons via rotor-stator molecular interfaces: Combined ab initio and model study
Petreska, Irina; Ohanesjan, Vladimir; Pejov, Ljupčo; Kocarev, Ljupčo
2016-07-01
Tunneling of electrons through rotor-stator anthracene aldehyde molecular interfaces is studied with a combined ab initio and model approach. Molecular electronic structure calculated from first principles is utilized to model different shapes of tunneling barriers. Together with a rectangular barrier, we also consider a sinusoidal shape that captures the effects of the molecular internal structure more realistically. Quasiclassical approach with the Simmons' formula for current density is implemented. Special attention is paid on conformational dependence of the tunneling current. Our results confirm that the presence of the side aldehyde group enhances the interesting electronic properties of the pure anthracene molecule, making it a bistable system with geometry dependent transport properties. We also investigate the transition voltage and we show that conformation-dependent field emission could be observed in these molecular interfaces at realistically low voltages. The present study accompanies our previous work where we investigated the coherent transport via strongly coupled delocalized orbital by application of Non-equilibrium Green's Function Formalism.
We have studied the correlation between chemical composition, structure, chemical bonding and elastic properties of amorphous B6O based solids using ab initio molecular dynamics. These solids are of different chemical compositions, but the elasticity data appear to be a function of density. This is in agreement with previous experimental observations. As the density increases from 1.64 to 2.38 g cm-3, the elastic modulus increases from 74 to 253 GPa. This may be understood by analyzing the cohesive energy and the chemical bonding of these compounds. The cohesive energy decreases from -7.051 to -7.584 eV/atom in the elastic modulus range studied. On the basis of the electron density distributions, Mulliken analysis and radial distribution functions, icosahedral bonding is the dominating bonding type. C and N promote cross-linking of icosahedra and thus increase the density, while H hinders the cross-linking by forming OH groups. The presence of icosahedral bonding is independent of the density
Singh, Ram Sevak; Solanki, Ankit
2016-03-01
Silicon carbide nanotubes (SiCNTs) have received a great deal of scientific and commercial interest due to their intriguing properties that include high temperature stability and electronic properties. For their efficient and widespread applications, tuning of electronic properties of SiCNTs is an attractive study. In this article, electronic properties of sulphur doped (S-doped) zigzag (9 , 0) SiCNT is investigated by ab initio calculations based on density functional theory (DFT). Energy band structures and density of states of fully optimized undoped and doped structures with varying dopant concentration are calculated. S-doped on C-site of the nanotube exhibits a monotonic reduction of energy gap with increase in dopant concentration, and the nanotube transforms from semiconductor to metal at high dopant concentration. In case of S-doped on Si-site doping has less influence on modulating electronic structures, which results in reduction of energy gap up to a moderate doping concentration. Importantly, S preferential substitutes of Si-sites and the nanotube with S-doped on Si-site are energetically more stable as compared to the nanotube with S-doped on C-site. The study of tunable electronic properties in S-doped SiCNT may have potential in fabricating nanoelectronic devices, hydrogen storage and gas sensing applications.
Numerical study of two-photon ionization of helium using an ab initio numerical framework
Few-photon-induced breakup of helium is studied using a newly developed ab initio numerical framework for solving the six-dimensional time-dependent Schroedinger equation. We present details of the method and calculate (generalized) cross sections for the process of two-photon nonsequential (direct) double ionization at photon energies ranging from 39.4 to 54.4 eV, a process that has been very much debated in recent years and is not yet fully understood. In particular, we have studied the convergence property of the total cross section in the vicinity of the upper threshold (∼ 54.4 eV) versus the pulse duration of the applied laser field. We find that the cross section exhibits an increasing trend near the threshold, as has also been observed by others, and show that this rise cannot solely be attributed to an unintended inclusion of the sequential two-photon double ionization process caused by the bandwidth of the applied field.
Ab initio theoretical comparative study of magnetic coupling in KNiF3 and K2NiF4s
Moreira, Ibério de Pinho Ribeiro; Illas i Riera, Francesc
1997-01-01
The origin of magnetic coupling in KNiF3 and K2 NiF4 is studied by means of an ab initio cluster model approach. By a detailed study of the mapping between eigenstates of the exact nonrelativistic and spin model Hamiltonians it is possible to obtain the magnetic coupling constant J and to compare ab initio cluster-model values with those resulting from ab initio periodic Hartree-Fock calculations. This comparison shows that J is strongly determined by two-body interactions; this is a surprisi...
Ab initio study of a Y-doped Σ31 grain boundary in alumina
2008-01-01
The atomic structures and energetics of clean and Y-doped general grain boundary (GB) Σ31/(0001) models in α-Al2O3 are studied by a series of high precision ab initio calculations. A large supercell with 700 atoms and periodic boundary conditions is adopted for undoped and Y-doped GB with different substitution sites and con-centrations. It is shown that Y atoms preferably segregate to the central column of the 7-member Al ring. This is explained as more favorable bond formation for Y in this position and lower GB energy. The calculated GB formation energy for the clean and Y-doped cases is respectively 3.99 and 3.67 J/m2. On the average, the GB region in Σ31 has a slightly lower charge density than the bulk crystalline region. In addtition, the GB induces a long ranged asymmetric electrostatic potential distri-bution on each side of the grain boundary.
Ab initio kinetics studies of hydrogen atom abstraction from methyl propanoate.
Tan, Ting; Yang, Xueliang; Ju, Yiguang; Carter, Emily A
2016-02-14
The kinetics of hydrogen abstraction by five radicals (H, CH3, O((3)P), OH, and HO2) from a biodiesel surrogate, methyl propanoate (MP), is theoretically investigated. We employ high-level ab initio quantum chemistry methods, coupled-cluster singles and doubles with perturbative triples correction (CCSD(T)) and multi-reference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and obtain chemically accurate reaction energetics. Overall, MRSDCI + DS predicts comparable energetics to CCSD(T) for MP + H/CH3/O/OH. The rate constants are computed using transition state theory (TST-Rice-Ramsperger-Kassel-Marcus theory) in conjunction with the separable-hindered-rotor approximation, variable reaction coordinate TST, and the multi-structure all-structure (MS-AS) approach. A simplified method, semi-multi-structure, is also employed for MP + OH/HO2, and the rate coefficients with this less expensive method are in good agreement with the results obtained with the MS-AS method. The fitted modified Arrhenius expressions are provided over a temperature range of 250 to 2000 K. The predicted rate coefficients for MP + OH agree remarkably well with experimental data over a wide temperature range. Branching ratio analysis of all the studied reactions shows that abstractions of the secondary H atoms within MP are expected to dominate the consumption of fuel at low temperatures, and the contributions of abstractions from the two methyl groups increase with temperature for all abstracting radicals. PMID:26796249
Yoo, Su-Hyun; Lee, Ji-Hwan; Delley, Bernard; Soon, Aloysius
2014-09-14
A first-principles description and prediction of brominated nanocrystals of Pd is presented. In particular, we conducted an extensive study of the adsorption behaviour of Br on various Pd surfaces (including both low and high Miller-index surfaces) as a function of its surface coverage. By coupling our calculated surface energies with ab initio (electrochemical) thermodynamics and the Gibbs-Wulff shape model, we find that the relative stability of the Pd surfaces is strongly modified by Br, allowing high Miller-index surfaces of Pd (namely the (210) surface) to become competitively favourable at moderate concentrations of Br. We also show that Pd nanoparticles assume a cube-like crystal shape at high concentrations of Br, exposing mainly the (100) facets with a Br surface coverage of 0.5 ML. This not only confirms and explains recent solution synthesis results, but also provides a quantitative atomic picture of the exposed surface facets, which is crucial in understanding the local surface chemistry of shape-controlled nanoparticles for better nanocatalyst design. PMID:25075669
Ab initio computational study of vincristine as a biological active compound: NMR and NBO analyses
Shiva Joohari
2015-06-01
Full Text Available Vincristine is a biological active alkaloid that has been used clinically against a variety of neoplasms. In the current study we have theoretically investigated the magnetic properties of titled compound to predict physical and chemical properties of vincristine as a biological inhibitor. Ab initio computation using HF and B3LYP with 3-21G(d and 6-31G(d level of theory have been performed and then magnetic shielding tensor (, ppm, shielding asymmetry (, magnetic shielding anisotropy (aniso, ppm, the skew of a tensor (K, chemical shift anisotropy ( and chemical shift ( were calculated to indicate the details of the interaction mechanism between microtubules and vincristine. Moreover, EHOMO, ELUMO and Ebg were evaluated. The maximum and minimum values of Ebg were found in HF/3-21g and B3LYP/3-21g respectively. It was also uggested that O24, O37, O49 and O55 with minimum values of iso, are active sites of titled compound. Furthermore the calculated chemical shifts were compared with experimental data in DMSO and CDCl3 solvents.
ab initio Studies on Molecular Conductor (BEDSe-TTF)2[Fe(CN)5NO
YAO Kai-Lun; TU Hai-Bo; WANG Wei-Zhong
2001-01-01
In this paper the ab initio study using pseudopotential plane wave method with the local spin density functional approximation is presented for the molecular conductor (BEDSe-TTF)2[Fe(CN)5NO]. The mean electronic density distributions are obtained, and we find that the extended π orbital of the selenium does not affect the properties of material as assumed in other papers and the "side-by-side" type S...S interaction is the primary interaction between donors. From band structure calculations we analyze the influence of the NO groups on the electronic structure and magnetic properties of molecule. It is shown that the itinerant electrons important to electronic properties in these types of hybrids are delocalized electrons contributed by NO groups, instead of by the 3d electrons of Fe. Additionally, we have found that the localized magnetic moment is also contributed by the NO groups in this molecular conductor. From total energy calculations the molecular structure with the lowest energy is found due to the interaction between split spins, and the particular positions of the NO groups are obtained.
Ha, Viet-Anh; Waroquiers, David; Rignanese, Gian-Marco; Hautier, Geoffroy
2016-05-01
Transparent conducting oxides (TCOs) are essential to many technologies. These materials are doped (n- or p-type) oxides with a large enough band gap (ideally >3 eV) to ensure transparency. However, the high carrier concentration present in TCOs leads additionally to the possibility for optical transitions from the occupied conduction bands to higher states for n-type materials and from lower states to the unoccupied valence bands for p-type TCOs. The "second gap" formed by these transitions might limit transparency, and a large second gap has been sometimes proposed as a design criteria for high performance TCOs. Here, we study the influence of this second gap on optical absorption using ab initio computations for several well-known n- and p-type TCOs. Our work demonstrates that most known n-type TCOs do not suffer from second gap absorption in the visible even at very high carrier concentrations. On the contrary, p-type oxides show lowering of their optical transmission for high carrier concentrations due to second gap effects. We link this dissimilarity to the different chemistries involved in n- versus typical p-type TCOs. Quantitatively, we show that second gap effects lead to only moderate loss of transmission (even in p-type TCOs) and suggest that a wide second gap, while beneficial, should not be considered as a needed criteria for a working TCO.
Monteseguro, V. [Departamento de Física and MALTA Consolider Team, Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife (Spain); Rodríguez-Hernández, P.; Muñoz, A., E-mail: amunoz@ull.es [Departamento de Física and MALTA Consolider Team, Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife (Spain); Instituto de Materiales y Nanotecnología. Universidad de La Laguna, 38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife (Spain)
2015-12-28
The structural, elastic, and vibrational properties of yttrium aluminum garnet Y{sub 3}Al{sub 5}O{sub 12} are studied under high pressure by ab initio calculations in the framework of the density functional theory. The calculated ground state properties are in good agreement with the available experimental data. Pressure dependences of bond length and bulk moduli of the constituent polyhedra are reported. The evolution of the elastic constants and the major elastic properties, Young and shear modulus, Poisson's ratios, and Zener anisotropy ratio, are described. The mechanical stability is analyzed, on the light of “Born generalized stability criteria,” showing that the garnet is mechanically unstable above 116 GPa. Symmetries, frequencies, and pressure coefficients of the Raman-active modes are discussed on the basis of the calculated total and partial phonon density of states, which reflect the dynamical contribution of each atom. The relations between the phonon modes of Y{sub 3}Al{sub 5}O{sub 12} and the internal and external molecular modes of the different polyhedra are discussed. Infrared-active modes, as well as the silent modes, and their pressure dependence are also investigated. No dynamical instabilities were found below 116 GPa.
Ab initio study of the magnetostructural properties of MnAs
Rungger, Ivan; Sanvito, Stefano
2006-07-01
The magnetic and structural properties of MnAs are studied with ab initio methods and by mapping total energies onto a Heisenberg model. The stability of the different phases is found to depend mainly on the volume and on the amount of magnetic order, confirming previous experimental findings and phenomenological models. It is generally found that for large lattice constants the ferromagnetic state is favored, whereas for small lattice constants different antiferromagnetic states can be stabilized. In the ferromagnetic state the structure with minimal energy is always hexagonal, whereas it becomes orthorhombically distorted if there is an antiferromagnetic alignment of the magnetic moments in the hexagonal plane. For the paramagnetic state the stable cell is found to be orthorhombic up to a critical lattice constant of about 3.7Å , above which it remains hexagonal. This leads to the second-order structural phase transition between paramagnetic states at about 400K , where the lattice parameter increases above this critical value with rising temperature due to the thermal expansion. We also evaluate the magnetic susceptibility as a function of temperature, from which a semiquantitative description of the MnAs phase diagram emerges.
Marjan Rafiee
2015-09-01
Full Text Available Tyrosinase is a multifunctional copper-containing enzyme. It can catalyze two distinct reactions of melanin synthesis and benzaldehyde derivatives, which are potential tyrosinase inhibitors. To find the relationships between charge distributions of benzaldehyde and their pharmaceutical behavior, the present study aimed at investigating nuclear quadrupole coupling constants of quadrupolare nuclei in the functional benzaldehyde group and calculating some its derivatives. In addition, the differences between the electronic structures of various derivatives of this depigmenting drug were examined. All ab initio calculations were carried out using Gaussian 03. The results predicted benzaldehyde derivatives to be bicentral inhibitors; nevertheless, the oxygen or hydrogen contents of the aldehyde group were not found to be the only active sites. Furthermore with the presence of the aldehyde group, the terminal methoxy group in C4 was found to contribute to tyrosinase inhibitory activities. In addition, an oxygen atom with high charge density in the side chain was found to play an important role in its inhibitory effect.
Ab initio studies on the lattice thermal conductivity of silicon clathrate frameworks II and VIII
Härkönen, Ville J.; Karttunen, Antti J.
2016-01-01
The lattice thermal conductivities of silicon clathrate frameworks II and VIII are investigated by using ab initio lattice dynamics and an iterative solution of the linearized Boltzmann transport equation (BTE) for phonons. Within the temperature range 100-350 K, the clathrate structures II and VIII were found to have lower lattice thermal conductivity values than the silicon diamond structure (d -Si) by factors of 1/2 and 1/3, respectively. The main reason for the lower lattice thermal conductivity of the clathrate structure II in comparison to d -Si was found to be the harmonic phonon spectra, while in the case of the clathrate structure VIII, the difference is mainly due to the harmonic phonon spectra and partly due to the shorter relaxation times of phonons. In the studied clathrate frameworks, the anharmonic effects have larger impact on the lattice thermal conductivity than the size of the unit cell. For the structure II, the predicted lattice thermal conductivity differs approximately by a factor of 20 from the previous experimental results obtained for a polycrystalline sample at room temperature.
Ab initio computational studies on molecular conformation of N-methyl-glyphosate
Kaliannan, P.; Naseer Ali, M. Mohamed; Venuvanalingam, P.
Conformational analysis of N-methyl-glyphosate has been carried out using an ab initio molecular orbital (MO) method at the HF/3-21G* levels of theory and the results are compared with the results of a previously studied compound, namely glyphosate. The potential energy surface of the molecule obtained by varying the central torsion angles (Φ, Ψ) was investigated in detail. Fourteen conformers with 5 kcal mol-1 energy cut-off have been selected from the potential energy surface for geometry optimization to locate the true minimum on the conformational space. The minimum has been found to be at (-62°, 110°) for the central torsion angles. This conformation is stabilized by hydrogen bond interactions (O-H···O and C-H···O) and the interactions due to protons nearer to each other. This cationic field leads to the formation of a hydrophobic patch in this structure, as well as in the structures closer to the global minimum. This patch may destabilize the favourable interaction of N-methyl-glyphosate with the surrounding amino acid residues in the binding cavity as they form the cationic field throughout the glyphosate binding region.
We use ab initio methods to study the binding, diffusion, and magnetic properties of cobalt atom embedded in graphene vacancies. We investigate the diffusion of Co-monovacancy (Co-MV) and Co-divacancy (Co-DV) defect complexes, and determine the minimum energy path (MEP), as well as the activation energy barrier of migration. We obtained similar activation energy barriers, of ∼5.8 eV, for Co-MV and Co-DV diffusion, respectively. Our calculations also suggest that, at electron–irradiation energy of 200 keV as used in a related experiment, the maximum energy transfer to the Co atom, of approximately 9.0 eV is sufficiently high to break metal-carbon bonding. The incident electron energy is also high enough to displace graphene's carbon atoms from their lattice positions. The breaking of metal-carbon bonding and the displacement of graphene atoms may act to facilitate the migration of Co. We conclude therefore that the detrapping and diffusion of cobalt as observed experimentally is likely to be radiation-induced, similar to what has been observed for Au and Fe in electron-irradiated graphene. Furthermore, we show that Co migration in graphene is such that its magnetic moment varies along the diffusion path. The magnetic moment of Co is consistently higher in Co-DV diffusion when compared to that of Co-MV diffusion
Ab initio pseudopotential studies of cubic BC2N under high pressure
We present the results of a systematic study of the structural, electronic, and vibrational properties of various cubic BC2N phases under high pressure. Ab initio pseudopotential total-energy and phonon calculations have been carried out to examine the changes in the structural parameters, bonding behaviours, band structures, and dynamic instabilities caused by phonon softening in these phases. We find that an experimentally synthesized high-density phase of cubic BC2N exhibits outstanding stability in the structural and electronic properties up to very high pressures. On the other hand, another experimentally identified phase with lower density and lower symmetry undergoes a dramatic structural transformation with a volume and bond-length collapse and a concomitant semi-metal to semiconductor transition. A third phase is predicted to be favourable over the above-mentioned lower-density phase by the enthalpy calculations. However, the dynamic phonon calculations reveal that it develops imaginary phonon modes and, therefore, is unstable in the experimental pressure range. The calculations indicate that its synthesis may be achieved at reduced pressures. These results provide a comprehensive understanding for the high-pressure behaviour of the cubic BC2N phases and reveal their interesting properties that can be verified by experiments
AB INITIO HF AND DFT STUDIES ON MOLECULAR STRUCTURE AND VIBRATIONAL ANALYSIS OF 2,5-DIBROMOPYRIDINE
ÇIRAK, Çağrı; KOÇ, Nurettin
2014-01-01
Theoretical study on molecular structure and vibrational spectra of 2,5-dibromopyridine (2,5-DBP) have been investigated. The optimized geometry, theoretical vibration frequencies and intensities were calculated by using ab initio Hartree-Fock and density functional B3LYP method with 6-31G(d,p) basis sets. The vibrational analysis of title molecule was done and its optimized geometry parameters (bond lengths and bond angles) were given. Scaled theoretical frequencies have been compared with e...
Silvestrelli, P.-L.; Alavi, A; Parrinello, M.; Frenkel, D
1997-01-01
The method of ab initio molecular dynamics, based on finite-temperature density-functional theory, is used to simulate laser heating of crystalline silicon. We found that a high concentration of excited electrons dramatically weakens the covalent bonding. As a result the system undergoes a melting transition to a metallic state. We studied several structural, dynamical, electronic, and bonding properties of this phase of silicon. In contrast to ordinary liquid silicon, this liquid is characte...
On the hierarchical parallelization of ab initio simulations
Ruiz-Barragan, Sergi; Shiga, Motoyuki
2016-01-01
A hierarchical parallelization has been implemented in a new unified code PIMD-SMASH for ab initio simulation where the replicas and the Born-Oppenheimer forces are parallelized. It is demonstrated that ab initio path integral molecular dynamics simulations can be carried out very efficiently for systems up to a few tens of water molecules. The code was then used to study a Diels-Alder reaction of cyclopentadiene and butenone by ab initio string method. A reduction in the reaction energy barrier is found in the presence of hydrogen-bonded water, in accordance with experiment.
The electronic structure of wurtzite and zincblende AlN: an ab initio comparative study
This work deals with the electronic properties, in different crystal phases, of AlN (wurtzite and zincblende) compounds computed using an all electron ab initio linearized augmented plane wave method. Results include band structure, total and partial density of states, charge density and the ionicity factor. Most of the calculated band parameters, of direct bandgap, total- and upper-valence bandwidths and antisymmetric gap for wurtzite-AlN are close to those of c-AlN to within 1%. The charge distributions have similar features, meaning that AlN has the same ionicity factor in both structures. (author)
Low-lying electronic states of nitrosyl cyanide (NCNO): An ab initio MCHF study
Ab initio multiconfiguration Hartree--Fock calculations of the electronic structure of several low-lying electronic states of nitrosyl cyanide (NCNO) are reported. The essential features of the electronic structure of these states were analyzed to provide a qualitative correlation diagram for the dissociation process NCNO → NC+NO. It is found that the four lowest-lying states 1/sup ,/3A' and 1/sup ,/3A'' are connected directly to ground state products. Excitation energies and geometrical parameters for these states and for the 2- 1A'' and 2- 1A' states are presented. Implications of these findings for NCNO photodissociation processes are discussed
Ab initio study of beryllium-decorated fullerenes for hydrogen storage
Lee, Hoonkyung; Huang, Bing; Duan, Wenhui; Ihm, Jisoon
2010-01-01
We have found that a beryllium (Be) atom on nanostructured materials with H2 molecules generates a Kubas-like dihydrogen complex [H. Lee et al. arXiv:1002.2247v1 (2010)]. Here, we investigate the feasibility of Be-decorated fullerenes for hydrogen storage using ab initio calculations. We find that the aggregation of Be atoms on pristine fullerenes is energetically preferred, resulting in the dissociation of the dihydrogen. In contrast, for boron (B)-doped fullerenes, Be atoms prefer to be ind...
Ab initio studies of ionization potentials of hydrated hydroxide and hydronium
Swartz, Charles W
2013-01-01
The ionization potential distributions of hydrated hydroxide and hydronium are computed with many-body approach for electron excitations with configurations generated by {\\it ab initio} molecular dynamics. The experimental features are well reproduced and found to be closely related to the molecular excitations. In the stable configurations, the ionization potential is mainly perturbed by water molecules within the first solvation shell. On the other hand, electron excitation is delocalized on both proton receiving and donating complex during proton transfer, which shifts the excitation energies and broadens the spectra for both hydrated ions.
Experimental and ab initio study of the photofragmentation of DNA and RNA sugars
The photoelectron-photoion-photoion coincidence method is used to measure the photodissociation of doubly charged D-ribose (C5H10O5), the RNA sugar molecules, and 2-deoxy-D-ribose (C5H10O4), the DNA sugar molecules, following normal Auger decay after initial C 1s and O 1s core ionizations. The fragment identification is facilitated by measuring isotopically labeled D-ribose, such as D-ribose deuterated at C(1), and with 13C at the C(5) position. Ab initio quantum chemistry calculations are used to gain further insight into the abundant appearance of the CHO+ fragment.
Ab-initio study of magnetic properties and phase transitions in Ga (Mn) N with Monte Carlo approach
Sbai, Y.; Ait Raiss, A.; Salmani, E.; Bahmad, L.; Benyoussef, A.
2015-12-01
On the basis of ab-initio calculations and Monte Carlo simulations the magnetic and electronic properties of Gallium nitride (GaN) doped with the transition metal Manganese (Mn) were studied. The ab initio calculations were done using the AKAI-KKR-CPA method within the Local Density Approximation (LDA) approximation. We doped our Diluted Magnetic Semiconductor (DMS), with different concentrations of magnetic impurities Mn and plotted the density of state (DOS) for each one. Showing a half-metallic behavior and ferromagnetic state especially for Ga0.95Mn0.05N making this DMS a strong candidate for spintronic applications. Moreover, the magnetization and susceptibility of our system as a function of the temperature has been calculated and give for various system size L to study the size effect. In addition, the transition temperature was deduced from the peak of the susceptibility. The Ab initio results are in good agreement with literature especially for (x=0.05) of Mn which gives the most interesting results.
Revealing halogen bonding interactions with anomeric systems: an ab initio quantum chemical studies.
Lo, Rabindranath; Ganguly, Bishwajit
2015-02-01
A computational study has been performed using MP2 and CCSD(T) methods on a series of O⋯X (X=Br, Cl and I) halogen bonds to evaluate the strength and characteristic of such highly directional noncovalent interactions. The study has been carried out on a series of dimeric complexes formed between interhalogen compounds (such as BrF, BrCl and BrI) and oxygen containing electron donor molecule. The existence and consequences of the anomeric effect of the electron donor molecule has also been investigated through an exploration of halogen bonding interactions in this halogen bonded complexes. The ab initio quantum chemical calculations have been employed to study both the nature and directionality of the halogen molecules toward the sp(3) oxygen atom in anomeric systems. The presence of anomeric nO→σ*CN interaction involves a dominant role for the availability of the axial and equatorial lone pairs of donor O atom to participate with interhalogen compounds in the halogen-bonded complexes. The energy difference between the axial and equatorial conformers with interhalogen compounds reaches up to 4.60 kJ/mol, which however depends upon the interacting halogen atoms and its attaching atoms. The energy decomposition analysis further suggests that the total halogen bond interaction energies are mainly contributed by the attractive electrostatic and dispersion components. The role of substituents attached with the halogen atoms has also been evaluated in this study. With the increase of halogen atom size and the positive nature of σ-hole, the halogen atom interacted more with the electron donor atom and the electrostatic contribution to the total interaction energy enhances appreciably. Further, noncovalent interaction (NCI) studies have been carried out to locate the noncovalent halogen bonding interactions in real space. PMID:25522359
Origin of the Hadži ABC structure: An ab initio study
Medium and strong hydrogen bonds are well known to give rise to broad features in the vibrational spectrum often spanning several hundred wavenumbers. In some cases, these features can span over 1000 cm−1 and even contain multiple broad peaks. One class of strongly hydrogen-bonded dimers that includes many different phosphinic, phosphoric, sulfinic, and selenic acid homodimers exhibits a three-peaked structure over 1500 cm−1 broad. This unusual feature is often referred to as the Hadži ABC structure. The origin of this feature has been debated since its discovery in the 1950s. Only a couple of theoretical studies have attempted to interpret the origin of this feature; however, no previous study has been able to reproduce this feature from first principles. Here, we present the first ab initio calculation of the Hadži ABC structure. Using a reduced dimensionality calculation that includes four vibrational modes, we are able to reproduce the three-peak structure and much of the broadness of the feature. Our results indicate that Fermi resonances of the in-plane bend, out-of-plane bend, and combination of these bends play significant roles in explaining this feature. Much of the broadness of the feature and the ability of the OH stretch mode to couple with many overtone bending modes are captured by including an adiabatically separated dimer stretch mode in the model. This mode modulates the distance between the monomer units and accordingly the strength of the hydrogen-bonds causing the OH stretch frequency to shift from 2000 to 3000 cm−1. Using this model, we were also able to reproduce the vibrational spectrum of the deuterated isotopologue which consists of a single 500 cm−1 broad feature. Whereas previous empirical studies have asserted that Fermi resonances contribute very little to this feature, our study indicates that while not appearing as a separate peak, a Fermi resonance of the in-plane bend contributes substantially to the feature
Arjunan, V.; Mohan, S.; Ravindran, P.; Mythili, C. V.
2009-05-01
The Fourier transform infrared (FTIR) and FT-Raman spectra of 7-bromo-5-chloro-8-hydroxyquinoline (BCHQ) have been measured in the range 4000-400 and 4000-100 cm -1, respectively. Complete vibrational assignment and analysis of the fundamental modes of the compound were carried out using the observed FTIR and FT-Raman data. The geometry was optimised without any symmetry constrains using the DFT/B3LYP and HF methods with 6-31G** basis set. The vibrational frequencies which were determined experimentally are compared with those obtained theoretically from ab initio HF and density functional theory (DFT) gradient calculations employing the HF/6-31G** and B3LYP/6-31G** methods for the optimised geometry of the compound. The structural parameters and normal modes of vibration obtained from HF and DFT methods are in good agreement with the experimental data. Normal coordinate analysis was also carried out with ab initio force fields utilising Wilson's FG matrix method.
Ab initio study on electron excitation and electron transfer in tryptophan-tyrosine system
In this article, ab initio calculation has been performed to evaluate the transition energy of electronic excitation in tryptophan and tyrosine by using semiempirical molecular orbital method AM1 and complete active space self-consistent field method. The solvent effect has been considered by means of the conductor-like screening model. After geometric optimizations of isolated tryptophan and tyrosine, and their corresponding radicals and cations, reaction heat of these electron transfer reactions have been obtained by the means of complete active space self-consistent field method. The transition energies from the ground state, respectively, to the lowest excited state and to the lowest triplet state of these two amino acids are also calculated and compared with the experimentally observed values. The ionization potential and electron affinity are also calculated for tryptophan and tyrosine employing Koopmans' theorem and ab initio calculation. Compared with the experimental measurements, the theoretical results are found satisfactory. Theoretical results give good explanations on the experimental phenomena that N3· can preferably oxide the side chain of tryptophan residue and then the electron transfer from tyrosine residue to tryptophan residue follows in peptides involving tryptophan and tyrosine
An ab initio cluster study of the structure of the Si(001) surface
Ab initio calculations, employing double zeta plus polarization (DZP) basis sets and generalized valence bond (GVB) wave functions, have been performed on clusters of varying size, to investigate the utility of such clusters as prototypes for the study of silicon surfaces, and to investigate the effect of the level of theory used on predicted results. This work builds on landmark papers by Goddard in 1982 and Paulus in 1998 that demonstrate that a single reference wave function description of the silicon dimer bond is incorrect, and that a multireference description results in a symmetric dimer in a silicon cluster containing one dimer. In this work, it is shown that the imposition of arbitrary geometrical constraints (fixing subsurface atoms at lattice positions) on cluster models of the Si(100) surface can also lead to nonphysical results. Calculations on the largest clusters, without geometrical constraints, reveal that surface rearrangement due to dimer bond formation is ''felt'' several layers into the bulk. The predicted subsurface displacements compare favorably to experiment. Thus, small clusters, such as Si9H12, cannot adequately represent bulk behavior. Vibrational analysis shows that dimer buckling modes require minimal excitation energy, so the experimental observation of buckled dimers on silicon surfaces may reflect the ease with which a symmetric dimer can be perturbed from its minimum energy structure. In the study of surface reconstruction and relaxation, and the associated issue of the buckling of dimer surfaces, it is critical to use adequate wave functions. As shown in this work and previously by Goddard and Paulus, this generally means that multireference treatments are needed to correctly treat the dangling bonds. (c) 2000 American Institute of Physics
Diffusion within α-CuI studied using ab initio molecular dynamics simulations
Mohn, Chris E.; Stølen, Svein; Hull, Stephen
2009-08-01
The structure and dynamics of superionic α-CuI are studied in detail by means of ab initio Born-Oppenheimer molecular dynamics simulations. The extreme cation disorder and a soft immobile face centred cubic sublattice are evident from the highly diffuse atomic density profiles. The Cu-Cu pair distribution function and distribution of Cu-I-Cu bond angles possess distinct peaks at 2.6 Å and 60° respectively, which are markedly lower than the values expected from the average cationic density, pointing to the presence of pronounced short-range copper-copper correlations. Comparison with lattice static calculations shows that these correlations and the marked shift in the cationic density profile in the lang111rang directions are associated with a locally distorted cation sublattice, and that the movements within the tetrahedral cavities involve rapid jumps into and out of shallow basins on the system potential energy surface. On average, the iodines are surrounded by three coppers within their first coordination shell, with the fourth copper being located in a transition zone between two neighbouring iodine cavities. However, time-resolved analysis reveals that the local structure actually involves a mixture of threefold-, fourfold- and fivefold-coordinated iodines. Examination of the ionic trajectories shows that the copper ions jump rapidly to nearest neighbouring tetrahedral cavities (aligned in the lang100rang directions) following a markedly curved trajectory and often involving short-lived (~1 ps) interstitial positions. The nature of the correlated diffusion underlying the unusually high fraction of coppers with short residence time can be attributed to the presence of a large number of 'unsuccessful' jumps and the likelihood of cooperative motion of pairs of coppers. The calculated diffusion coefficient at 750 K, DCu = 2.8 × 10-5 cm2 s-1, is in excellent agreement with that found experimentally.
Ab initio study of different structures of CaC: Magnetism, bonding, and lattice dynamics
Highlights: •Electronic structure and phonon calculations are performed on seven CaC structures. •The more covalent structures have lower energies and nonmagnetic ground state. •Ferromagnetism in the ionic phases comes from the sharp C p band at the Fermi level. •Tendency of C atoms for dimerization may lead to structural instabilities in CaC. •Nonmagnetic B33 CaC is stable in a wide range of temperatures and pressures. -- Abstract: On the basis of ab initio pseudopotential calculations, we study structural, magnetic, dynamical, and mechanical properties of the hypothetical CaC ionic compound in the rock-salt (RS), B2, zinc-blende (ZB), wurtzite (WZ), NiAs (NA), anti-NiAs (NA∗), and CrB (B33) structures. It is argued that the ZB, WZ, NA, and RS structures are more ionic while the NA∗, B2, and B33 structures are more covalent systems. As a result of that, the nonmagnetic B33–CaC is the energetically preferred system, while the more ionic structures prefer a ferromagnetic ground state with high Fermi level spin polarization. The observed ferromagnetism in the more ionic systems is attributed to the sharp partially filled p states of carbon atom in the system. In the framework of density functional perturbation theory, the phonon spectra of these systems are computed and the observed dynamical instabilities of the NA∗ and B2 structures are explained in terms of the covalent bonds between carbon atoms. The calculated Helmholtz and Enthalpy free energies indicate the highest stability of the B33 structure in a wide range of temperatures and pressures. Among the ferromagnetic structures, RS–CaC and ZB–CaC are reported, respectively, to be the most and the least metastable systems in various thermodynamics conditions. Several mechanical properties of the dynamically stable structures of CaC are determined from their phonon spectra
Suter, James L.; Kabalan, Lara; Khader, Mahmoud; Coveney, Peter V.
2015-11-01
Ab initio molecular dynamics simulations have been performed to gain an understanding of the interfacial microscopic structure and reactivity of fully hydrated clay edges. The models studied include both micropore and interlayer water. We identify acidic sites through dissociation mechanisms; the resulting ions can be stabilized by both micropore and interlayer water. We find clay edges possess a complex amphoteric behavior, which depends on the face under consideration and the location of isomorphic substitution. For the neutral (1 1 0) surface, we do not observe any dissociation on the timescale accessible. The edge terminating hydroxyl groups participate in a hydrogen bonded network of water molecules that spans the interlayer between periodic images of the clay framework. With isomorphic substitutions in the tetrahedral layer of the (1 1 0) clay edge, we find the adjacent exposed apical oxygen behaves as a Brönsted base and abstracts a proton from a nearby water molecule, which in turn removes a proton from an AlOH2 group. With isomorphic substitutions in the octahedral layer of the (1 1 0) clay edge the adjacent exposed apical oxygen atom does not abstract a proton from the water molecules, but increases the number of hydrogen bonded water molecules (from one to two). Acid treated clays are likely to have both sites protonated. The (0 1 0) surface does not have the same interfacial hydrogen bonding structure; it is much less stable and we observe dissociation of half the terminal SiOH groups (tbnd Sisbnd Osbnd H → tbnd Sisbnd O- + H+) in our models. The resulting anions are stabilized by solvation from both micropore and interlayer water molecules. This suggests that, when fully hydrated, the (0 1 0) surface can act as a Brönsted acid, even at neutral pH.
Ab initio studies on the adsorption and implantation of Al and Fe to nitride materials
The formation of transfer material products on coated cutting and forming tools is a major failure mechanism leading to various sorts of wear. To describe the atomistic processes behind the formation of transfer materials, we use ab initio to study the adsorption energy as well as the implantation barrier of Al and Fe atoms for (001)-oriented surfaces of TiN, Ti0.50Al0.50N, Ti0.90Si0.10N, CrN, and Cr0.90Si0.10N. The interactions between additional atoms and nitride-surfaces are described for pure adhesion, considering no additional stresses, and for the implantation barrier. The latter, we simplified to the stress required to implant Al and Fe into sub-surface regions of the nitride material. The adsorption energies exhibit pronounced extrema at high-symmetry positions and are generally highest at nitrogen sites. Here, the binary nitrides are comparable to their ternary counterparts and the average adhesive energy is higher (more negative) on CrN than TiN based systems. Contrary, the implantation barrier for Al and Fe atoms is higher for the ternary systems Ti0.50Al0.50N, Ti0.90Si0.10N, and Cr0.90Si0.10N than for their binary counterparts TiN and CrN. Based on our results, we can conclude that TiN based systems outperform CrN based systems with respect to pure adhesion, while the Si-containing ternaries exhibit higher implantation barriers for Al and Fe atoms. The data obtained are important to understand the atomistic interaction of metal atoms with nitride-based materials, which is valid not just for machining operations but also for any combination such as interfaces between coatings and substrates or multilayer and phase arrangements themselves
Study of the behaviour of cesium fission product in uranium dioxide by the ab initio method
The knowledge of the behaviour of fission products in the nuclear fuel is very important for safety considerations and for understanding the evolution of the fuel properties under irradiation. In this work, we focussed mainly on the behaviour of caesium in UO2 through ab initio studies of its solubility at point defects in the matrix, its diffusion and its contribution to the formation of solid phases in the fuel. The role of electronic correlation effects of the f electrons of uranium on these properties and on the description of the defect free crystal, is assessed. The formation energies of the main point defects are calculated and their concentration as a function of fuel stoichiometry and temperature is estimated. The migration barriers and migration paths for the self-diffusion of oxygen and uranium vacancies and oxygen interstitials in UO2 are discussed. The solubility of Cs is found to be very low in UO2 in agreement with experimental findings. The most favourable trapping sites are determined as a function of oxygen concentration in the fuel. Our results show that in the hyper-stoichiometric regime, the diffusion of Cs from its most favourable trapping site is limited by the uranium vacancy diffusion mechanism. We also considered the formation of the main solid phases of caesium resulting from its oxidation (Cs2O, Cs2O2, CsO2) and from its interaction with the fuel (Cs2UO4), with molybdenum (Cs2MoO4) and with the zirconium of the clad (Cs2ZrO3), since the formation of such phases, their solubility and their interdependence will affect the release of caesium. (author)
Crystal structure and magnetism in α -RuCl3 : An ab initio study
Kim, Heung-Sik; Kee, Hae-Young
2016-04-01
α -RuCl3 has been proposed recently as an excellent playground for exploring Kitaev physics on a two-dimensional (2D) honeycomb lattice. However, structural clarification of the compound has not been completed, which is crucial in understanding the physics of this system. Here, using ab initio electronic structure calculations, we study a full three-dimensional (3D) structure of α -RuCl3 , including the effects of spin-orbit coupling (SOC) and electronic correlations. The three major results are as follows: (i) SOC suppresses dimerization of Ru atoms, which exists in other Ru compounds such as isostructural Li2RuO3 , and makes the honeycomb closer to an ideal one. (ii) The nearest-neighbor Kitaev exchange interaction between the jeff=1 /2 pseudospin strongly depends on the Ru-Ru distance and the Cl position, originating from the nature of the edge-sharing geometry. (iii) The optimized 3D structure without electronic correlations has P 3 ¯1 m space-group symmetry independent of SOC, but including electronic correlation changes the optimized 3D structure to either C 2 /m or C m c 21 within 0.1 meV per formula unit (f.u.) energy difference. The reported P 3112 structure is also close in energy. The interlayer spin-exchange coupling is a few percent of the in-plane spin-exchange terms, confirming that α -RuCl3 is close to a 2D system. We further suggest how to increase the Kitaev term via tensile strain, which sheds light in realizing the Kitaev spin-liquid phase in this system.
Reaction mechanisms and kinetics of the iminovinylidene radical with NO: Ab initio study
Hsiao, Ming-Kai; Chung, Yi-Hua; Hung, Yu-Ming; Chen, Hui-Lung, E-mail: chl3@faculty.pccu.edu.tw [Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei 111, Taiwan (China)
2014-05-28
The nitric oxide (NO) is a notorious compound for polluting environment. Recent year, removing nitric oxide from the atmosphere becomes a focus of the investigation. In our work, we study the iminovinylidene (HNCC) radical reacted with NO molecule. The mechanism and kinetic for reaction of the HNCC radical with the NO molecule is investigated via considering the possible channels of the N and O atoms of NO attacking the N and C atoms of the HNCC based on the high level ab initio molecular orbital calculations in conjunction with variational TST and RRKM calculations. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level and their single-point energies are refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) method. The calculated potential energy surfaces indicated that energetically the most favorable channel for the HNCC + NO reaction was predicted to be the formation of HNC+CNO (P8) product via the addition reaction of the C atom of HNCC radical and the N atom of NO with the head to head orientation. To rationalize the scenario of the calculated results, we also employ the Fukui functions and HSAB theory to seek for a possible explanation. In addition, the reaction rate constants were calculated using VariFlex code, and the results show that the total rate coefficient, k{sub total}, at Ar pressure 760 Torr can be represented with an equation: k{sub total} = 6.433 × 10{sup −11} T {sup 0.100} exp(0.275 kcal mol{sup −1}/RT) at T = 298–3000 K, in units of cm{sup 3} molecule{sup −1} s{sup −1}.
Diffusion within α-CuI studied using ab initio molecular dynamics simulations
The structure and dynamics of superionic α-CuI are studied in detail by means of ab initio Born-Oppenheimer molecular dynamics simulations. The extreme cation disorder and a soft immobile face centred cubic sublattice are evident from the highly diffuse atomic density profiles. The Cu-Cu pair distribution function and distribution of Cu-I-Cu bond angles possess distinct peaks at 2.6 A and 60 deg. respectively, which are markedly lower than the values expected from the average cationic density, pointing to the presence of pronounced short-range copper-copper correlations. Comparison with lattice static calculations shows that these correlations and the marked shift in the cationic density profile in the (111) directions are associated with a locally distorted cation sublattice, and that the movements within the tetrahedral cavities involve rapid jumps into and out of shallow basins on the system potential energy surface. On average, the iodines are surrounded by three coppers within their first coordination shell, with the fourth copper being located in a transition zone between two neighbouring iodine cavities. However, time-resolved analysis reveals that the local structure actually involves a mixture of threefold-, fourfold- and fivefold-coordinated iodines. Examination of the ionic trajectories shows that the copper ions jump rapidly to nearest neighbouring tetrahedral cavities (aligned in the (100) directions) following a markedly curved trajectory and often involving short-lived (∼1 ps) interstitial positions. The nature of the correlated diffusion underlying the unusually high fraction of coppers with short residence time can be attributed to the presence of a large number of 'unsuccessful' jumps and the likelihood of cooperative motion of pairs of coppers. The calculated diffusion coefficient at 750 K, DCu = 2.8 x 10-5 cm2 s-1, is in excellent agreement with that found experimentally.
Chen, J; Ren, X.; Li, X Z; Alfè, D.; Wang, E
2014-01-01
The finite-temperature phase diagram of hydrogen in the region of phase IV and its neighborhood was studied using the ab initio molecular dynamics (MD) and the ab initio path-integral molecular dynamics (PIMD). The electronic structures were analyzed using the density-functional theory (DFT), the random-phase approximation, and the diffusion Monte Carlo (DMC) methods. Taking the state-of-the-art DMC results as benchmark, comparisons of the energy differences between structures generated from ...
Ab initio study of the low-lying electronic states of the CaO molecule
Graphical abstract: Highly correlated ab initio calculations have been performed for an accurate determination of the electronic structure of the low-lying electronic states of the CaO molecule. The computations are done using the aug-cc-pV5Z basis set for O and the cc-pCV5Z for Ca. The potential energy curves for the molecular states correlating to the lowest three asymptotes are calculated at the CASSCF level. The potential curves of the lowest five molecular states, X1Σ+, a3Π, A'1Π, b3Σ+ and A1Σ+, and the corresponding dipole moment functions have been determined using internally contracted multi-reference configuration interaction approaches. The spectroscopic constants associated with these electronic states are compared to experimental values. The corresponding electronic wavefunctions have also been analyzed using the dipole moment functions. Display Omitted Highlights: → The five lowest electronic states of Cao have been determined ab initio at a high level of accuracy. → Large active space, core-valence correlation and configuration interaction are required. → The multi-configurational nature of the electronic ground state is confirmed as well as its monovalent and divalent ionic nature using dipole moment analysis. → These interacting potentials will serve for future obtention of spin-rovibronic levels. - Abstract: Highly correlated ab initio calculations have been performed for an accurate determination of the electronic structure of the low-lying electronic states of the CaO molecule. The computations are done using the aug-cc-pV5Z basis set for O and the cc-pCV5Z for Ca. The potential energy curves for the molecular states correlating to the lowest three asymptotes are calculated at the CASSCF level. The potential curves of the lowest five molecular states, X1Σ+, a3Π, A'1Π, b3Σ+ and A1Σ+, and the corresponding dipole moment functions have been determined using internally contracted multi-reference configuration interaction approaches
Lucas, G
2006-10-15
The behaviour of silicon carbide under irradiation has been studied using classical and ab initio simulations, focusing on the nano scale elementary processes. First, we have been interested in the calculation of threshold displacement energies, which are difficult to determine both experimentally and theoretically, and also the associated Frenkel pairs. In the framework of this thesis, we have carried out simulations in classical and ab initio molecular dynamics. For the classical approach, two types of potentials have been used: the Tersoff potential, which led to non satisfactory results, and a new one which has been developed during this thesis. This potential allows a better modelling of SiC under irradiation than most of the empirical potentials available for SiC. It is based on the EDIP potential, initially developed to describe defects in silicon, that we have generalized to SiC. For the ab initio approach, the feasibility of the calculations has been validated and average energies of 19 eV for the C and 38 eV for the Si sublattices have been determined, close to the values empirically used in the fusion community. The results obtained with the new potential EDIP are globally in agreement with those values. Finally, the elementary processes involved in the crystal recovery have been studied by calculating the stability of the created Frenkel pairs and determining possible recombination mechanisms with the nudged elastic band method. (author)
Ab initio study of the electronic properties of the planar Ga5N5 cluster
Zheng Hao-Ping; Hao Jing-An
2005-01-01
The first-principles, all electron, ab initio calculations have been performed for an the amazing stable planar structure of Ga5N5 cluster based on the density functional theory. Electronic structure, Electron affinity, ionization potential, and binding energy are obtained. No spin magnetic moment is found. The results show that the planar structure of the Ga5N5 cluster is stable. It is found that for the planar structure of Ga5N5 cluster, three nitrogen atoms in the N3 subunit bind together with large electon transfer although no free N3 can exist. This may be important to the stability of the planar structure of the Ga5N5 cluster which has the lowest ground-state energy.
Ab initio study on the dynamics of furfural at the liquid-solid interfaces
Dang, Hongli; Xue, Wenhua; Shields, Darwin; Liu, Yingdi; Jentoft, Friederike; Resasco, Daniel; Wang, Sanwu
2013-03-01
Catalytic biomass conversion sometimes occurs at the liquid-solid interfaces. We report ab initio molecular dynamics simulations at finite temperatures for the catalytic reactions involving furfural at the water-Pd and water-Cu interfaces. We found that, during the dynamic process, the furan ring of furfural prefers to be parallel to the Pd surface and the aldehyde group tends to be away from the Pd surface. On the other hand, at the water-Cu(111) interface, furfural prefers to be tilted to the Cu surface while the aldehyde group is bonded to the surface. In both cases, interaction of liquid water and furfural is identified. The difference of dynamic process of furfural at the two interfaces suggests different catalytic reaction mechanisms for the conversion of furfural, consistent with the experimental investigations. Supported by DOE (DE-SC0004600). Simulations and calculations were performed on XSED's and NERSC's supercomputers
AB INITIO STUDY ON VALENCE INDICES AND REACTIVITIES OF SOME BORAENS
曹阳; 王友良
1991-01-01
In the pressnt paper, 3-21G ab initio molecular orbital calculations arc performed on diborane B2H6 and the substituted bridged-atom species H4B2X2(X=F, C1, OH, NH2, CH3),and these geometries are optimized with the energy gradient technique. According to the quantum chemical definition of atomic valence, the valences of the bridged-atoms are calculated to studtd the characteristion of the bridging bond B-X-B. Some larger boranes B4H10, B5H9, and B5H11 are also calculated to discuss the valence indices and analyze the reactiveities of the bridged-atoms.
Ab-initio Hartree-Fock study of tritium desorption from Li{sub 2}O
Taniguchi, Masaki; Tanaka, Satoru [Tokyo Univ. (Japan). Faculty of Engineering
1998-03-01
Dissociative adsorption of hydrogen on Li{sub 2}O (110) surface has been investigated with ab-initio Hartree-Fock quantum chemical calculation technique. Heat of adsorption and potential energy surface for H{sub 2} dissociative adsorption was evaluated by calculating the total energy of the system. Calculation results on adsorption heat indicated that H{sub 2} adsorption is endothermic. However, when oxygen vacancy exists adjacent to the adsorption sites, heat of adsorption energy became less endothermic and the activation energy required to dissociate the H-H bonding was smaller than that for the terrace site. This is considered to be caused by the excess charge localized near the defect. (author)
Ab initio study of beryllium-decorated fullerenes for hydrogen storage
Lee, Hoonkyung; Huang, Bing; Duan, Wenhui; Ihm, Jisoon
2010-04-01
We have found that a beryllium (Be) atom on nanostructured materials with H2 molecules generates a Kubas-like dihydrogen complex [Lee, Huang, Duan, and Ihm, Appl. Phys. Lett. 96, 143120 (2010)]. Here, we investigate the feasibility of Be-decorated fullerenes for hydrogen storage using ab initio calculations. We find that the aggregation of Be atoms on pristine fullerenes is energetically preferred, resulting in the dissociation of the dihydrogen. In contrast, for boron (B)-doped fullerenes, Be atoms prefer to be individually attached to B sites of the fullerenes, and a maximum of one H2 molecule binds to each Be atom in a form of dihydrogen with a binding energy of ˜0.3 eV. Our results show that individual dispersed Be-decorated B-doped fullerenes can serve as a room-temperature hydrogen storage medium.
Optoelectronic Properties, Elastic Moduli and Thermoelectricity of SrAlGa: An Ab Initio Study
Roshan, Ali; Murtaza, G.; Takagiwa, Y.; Khenata, R.; Haleem, Uddin; Ullah, H.; A. Khan, S.
2014-04-01
Half-Heusler compounds are an impressive class of materials with a huge potential for different applications such as in future energy, especially in the fields of thermoelectrics and solar cells. We present ab initio total energy calculations within the modified Becke—Johnson generalized gradient approximation (mBJ-GGA) to obtain the physical properties of SrAlGa compounds. The structural, elastic, acoustic, electronic, chemical bonding, optical, and thermoelectric properties are calculated and compared with the available calculation data. The SrAlGa is found to be a small-band-gap (0.125-0.175 eV) material, suitable for thermoelectric applications with a relatively high Seebeck coefficient. Also, SrAlGa has the potential in the optoelectronic applications due to high optical conductivity and reflectivity in the infrared and visible region of electromagnetic spectra.
Isomerism of OBe3F3+ cation: an ab initio study
Ab initio MP2/6-31G*/HF/6-31G*+ZPE(HF/6-31G*) calculations of the potential energy surface in the vicinity of stationary points and the pathways of intramolecular rearrangements between low-lying structures of the OBe3F3+ cation detected in the mass spectra of μ4-Be4O(CF3COO)6 were carried out. Ten stable isomers with di- and tricoordinate oxygen atoms were localized. The relative energies of six structures lie in the range 0-8 kcal mol-1 and those of the rest four structures lie in the range 20-40 kcal mol-1. two most favorable isomers are a planar C2, isomer and a pyramidal C3 isomer
Relaxation of the excited -(2-hydroxy benzylidene) aniline molecule: An ab initio and TD DFT study
Biswajit Chowdhury; Rina De; Pinaky Sett; Joydeep Chowdhury
2010-11-01
The photophysical behaviour of N-(2-hydroxy benzylidene) aniline or most commonly known as salicylideneaniline (SA) has been investigated using the ab initio and DFT levels of theory. The quantum chemical calculations show that the optimized non planar enol (1) form of the SA molecule is the most stable conformer in the ground state and is marked by the twisting of the phenolic and anilino rings of the molecule. The geometry optimizations and the subsequent frequency calculations of the excited singlet electronic states of the various tautomeric forms of SA molecule were performed with the CIS level of theory. A detail theoretical investigation on the relaxation dynamics of the SA molecule has been presented. Possible explanation on the excitation wavelength dependence of the photochromic yield of the molecule is also reported.
Ab initio and phenomenological studies of the static response of neutron matter
Buraczynski, Mateusz
2016-01-01
We investigate the problem of periodically modulated strongly interacting neutron matter. We carry out ab initio non-perturbative auxiliary-field diffusion Monte Carlo calculations using an external sinusoidal potential in addition to phenomenological two- and three-nucleon interactions. Several choices for the wave function ansatz are explored and special care is taken to extrapolate finite-sized results to the thermodynamic limit. We perform calculations at various densities as well as at different strengths and periodicities of the one-body potential. Our microscopic results are then used to constrain the isovector term from energy-density functional theories of nuclei at many different densities, while making sure to separate isovector contributions from bulk properties. Lastly, we use our results to extract the static density-density linear response function of neutron matter at different densities. Our findings provide insights into inhomogeneous neutron matter and are related to the physics of neutron-...
Ab initio study of pressure induced structural and electronic properties in TmPo
Makode, Chandrabhan, E-mail: cbmakode@gmail.com; Pataiya, Jagdish; Sanyal, Sankar P. [Department of Physics, Barkatullah University, Bhopal-462026 (India); Panwar, Y. S.; Aynyas, Mahendra [Department of Physics, C.S.A. Govt. P.G. College, Sehore-466001 (India)
2015-06-24
We report an ab initio calculation of pressure induced structural phase transition and electronic properties of Thulium Polonide (TmPo).The total energy as a function of volume is obtained by means of self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). It is found that TmPo is stable in NaCl-type (B{sub 1}-phase) structure to CsCl-type (B{sub 2}-phase) structure of this compound in the pressure range of 7.0 GPa. We also calculate the lattice parameter (a{sub 0}), bulk modulus (B{sub 0}), band structure and density of states. From energy diagram it is observed that TmPo exhibit metallic behavior. The calculated values of equilibrium lattice parameter and bulk modulus are in general good agreement.
Communication: Ab initio study of O4H+: A tracer molecule in the interstellar medium?
The structure and energetics of the protonated molecular oxygen dimer calculated via ab initio methods is reported. We find structures that share analogies with the eigen and zundel forms for the protonated water dimer although the symmetrical sharing of the proton is more prevalent. Analysis of different fragmentation channels show charge transfer processes which indicate the presence of conical intersections for various states including the ground state. An accurate estimate for the proton affinity of O4 leads to a significantly larger value (5.6 eV) than for O2 (4.4 eV), implying that the reaction H3+ + O4 → O4H+ + H2 is exothermic by 28 Kcal/mol as opposed to the case of O2 which is nearly thermoneutral. This opens up the possibility of using O4H+ as a tracer molecule for oxygen in the interstellar medium
Charge Transfer in FeO: A combined Molecular-Dynamics and Ab Initio Study
Molecular dynamics simulations and ab initio electronic structure calculations were carried out to determine the rate of charge transfer in stoichiometric w-stite (FeO). The charge transfer of interest occurs by II/III valence interchange between nearest-neighbor Fe atoms, with the Fe(III) constituting a ''hole'' electronic defect. There are two possible nearest-neighbor charge transfers in the FeO lattice, which occur between edge-sharing or corner-sharing FeO6 octahedra. Molecular dynamics simulations predict charge transfer rates of 3.7x1011 and 1.9x109 s-1 for the edge and corner transfers, respectively, in good agreement with those calculated using an ab initio cluster approach (1.6x1011 and 8.0x108 s-1, respectively). The calculated rates are also similar to those along basal and c-axis directions in hematite (?-Fe2O3) determined previously. Therefore, as is the case for hematite, w-stite is predicted to show anisotropic electrical conductivity. Our findings indicate that a rigid ion model does not give acceptable results, thus showing the need to account for the change in polarizability of the system upon charge transfer. Our model achieves this by using a simple mechanical shell model. By calculating the electronic coupling matrix elements for many transition state configurations obtained from the molecular dynamics simulations, we found evidence that the position of the bridging oxygen atoms can greatly affect the amount electronic coupling between the donor and acceptor states. Finally, we address the effect of oxygen vacancies on the charge transfer. It was found that an oxygen vacancy not only creates a driving force for holes to transport away from the vacancy (or equivalently for electrons to diffuse toward the vacancy) but also lowers the free energy barriers for charge transfer. In addition, the reorganization energy significantly differed from the non-defective case in a small radius around the defect
Ionization dynamics of the water trimer: A direct ab initio MD study
Tachikawa, Hiroto, E-mail: hiroto@eng.hokudai.ac.jp [Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628 (Japan); Takada, Tomoya [Department of Material Chemistry, Asahikawa National College of Technology, Syunkodai, Asahikawa 071-8142 (Japan)
2013-03-29
Highlights: ► We calculated ionization dynamics of water trimer. ► Direct ab initio molecular dynamics (MD) method is applied. ► Proton transfer dynamics were discussed. ► The proton transfer process calculated are well reproduced in recent experiments. - Abstract: Ionization dynamics of the cyclic water trimer (H{sub 2}O){sub 3} have been investigated by means of direct ab initio molecular dynamics (AIMD) method. Two reaction channels, complex formation and OH dissociation, were found following the ionization of (H{sub 2}O){sub 3}. In both channels, first, a proton was rapidly transferred from H{sub 2}O{sup +} to H{sub 2}O (time scale is ∼15 fs after the ionization). In complex channel, an ion–radical contact pair (H{sub 3}O{sup +}–OH) solvated by the third water molecule was formed as a long-lived H{sub 3}O{sup +}(OH)H{sub 2}O complex. In OH dissociation channel, the second proton transfer further takes place from H{sub 3}O{sup +}(OH) to H{sub 2}O (time scale is 50–100 fs) and the OH radical is separated from the H{sub 3}O{sup +}. At the same time, the OH dissociation takes place when the excess energy is efficiently transferred into the kinetic energy of OH radical. The OH dissociation channel is significantly minor, and almost all product channels were the complex formation. The reaction mechanism was discussed on the basis of theoretical results.
Graphical abstract: Halothane and enflurane form the blue-shifting hydrogen bonded C-H...complexes with benzene. The CCSD(T)/CBS calculated stabilization energies are about -10 kcal mol-1. - Abstract: For many years halothane and enflurane have been used clinically as volatile anaesthetics, however, their mechanism of action is still not fully understood. Recently, it has been suggested that they can act by a direct bonding to neuroreceptors containing the aromatic groups. In this work, the halothane...benzene and enflurane...benzene complexes were studied by the ab initio MP2 and CCSD(T) methods. All possible structures of the complexes were calculated by means of the counterpoise CP-corrected gradient optimization technique. It has been found that among these species, the C-H...π hydrogen bonded complexes are the most stable. The CCSD(T)/CBS calculated stabilization energies for halothane and enflurane complexes are: -10.56 and -9.72 kcal mol-1, respectively. The interaction energy is mainly dominated by the dispersion attraction. In the case of enflurane, the C-H bond shows a very small contraction (by -0.0008 A) upon complexation. This change is accompanied by the blue-shift (20 cm-1) of the C-H stretching frequency and an increase of the infrared intensity of the corresponding mode by 7 km mol-1. Similar results were obtained for the halothane complex: a small contraction of the C-H bond; an increase of the C-H stretching frequency by 11 cm-1 (blue-shift); and an increase of the infrared intensity by 37 km mol-1. In order to explain the nature of these effects, the halothane and enflurane molecules were studied in the electric field generated by benzene atoms, and Natural Bond Orbital (NBO) analyses were performed. The molecular dipole moments of these molecules were calculated with respect to the C-H bond changes. The positive dipole moment derivative obtained for halothane is in agreement with the literature data, while, in the case of enflurane, an unusual
Stacking stability of MoS2 bilayer: An ab initio study
Tao, Peng; Guo, Huai-Hong; Yang, Teng; Zhang, Zhi-Dong
2014-10-01
The study of the stacking stability of bilayer MoS2 is essential since a bilayer has exhibited advantages over single layer MoS2 in many aspects for nanoelectronic applications. We explored the relative stability, optimal sliding path between different stacking orders of bilayer MoS2, and (especially) the effect of inter-layer stress, by combining first-principles density functional total energy calculations and the climbing-image nudge-elastic-band (CI-NEB) method. Among five typical stacking orders, which can be categorized into two kinds (I: AA, AB and II: AA', AB', A'B), we found that stacking orders with Mo and S superposing from both layers, such as AA' and AB, is more stable than the others. With smaller computational efforts than potential energy profile searching, we can study the effect of inter-layer stress on the stacking stability. Under isobaric condition, the sliding barrier increases by a few eV/(ucGPa) from AA' to AB', compared to 0.1 eV/(ucGPa) from AB to [AB]. Moreover, we found that interlayer compressive stress can help enhance the transport properties of AA'. This study can help understand why inter-layer stress by dielectric gating materials can be an effective means to improving MoS2 on nanoelectronic applications.
Ab initio studies on the adsorption and implantation of Al and Fe to nitride materials
Riedl, H., E-mail: helmut.riedl@tuwien.ac.at [Christian Doppler Laboratory for Application Oriented Coating Development at the Institute of Materials Science and Technology, TU Wien, A-1040 Wien (Austria); Zálešák, J. [Erich Schmid Institute for Materials Science, Austria Academy of Science, A-8700 Leoben (Austria); Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700 Leoben (Austria); Arndt, M. [Oerlikon Balzers, Oerlikon Surface Solutions AG, LI-9496 Balzers (Liechtenstein); Polcik, P. [Plansee Composite Materials GmbH, D-86983 Lechbruck am See (Germany); Holec, D. [Christian Doppler Laboratory for Application Oriented Coating Development at the Institute of Materials Science and Technology, TU Wien, A-1040 Wien (Austria); Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700 Leoben (Austria); Mayrhofer, P. H. [Christian Doppler Laboratory for Application Oriented Coating Development at the Institute of Materials Science and Technology, TU Wien, A-1040 Wien (Austria); Institute of Materials Science and Technology, TU Wien, A-1040 Vienna (Austria)
2015-09-28
The formation of transfer material products on coated cutting and forming tools is a major failure mechanism leading to various sorts of wear. To describe the atomistic processes behind the formation of transfer materials, we use ab initio to study the adsorption energy as well as the implantation barrier of Al and Fe atoms for (001)-oriented surfaces of TiN, Ti{sub 0.50}Al{sub 0.50}N, Ti{sub 0.90}Si{sub 0.10}N, CrN, and Cr{sub 0.90}Si{sub 0.10}N. The interactions between additional atoms and nitride-surfaces are described for pure adhesion, considering no additional stresses, and for the implantation barrier. The latter, we simplified to the stress required to implant Al and Fe into sub-surface regions of the nitride material. The adsorption energies exhibit pronounced extrema at high-symmetry positions and are generally highest at nitrogen sites. Here, the binary nitrides are comparable to their ternary counterparts and the average adhesive energy is higher (more negative) on CrN than TiN based systems. Contrary, the implantation barrier for Al and Fe atoms is higher for the ternary systems Ti{sub 0.50}Al{sub 0.50}N, Ti{sub 0.90}Si{sub 0.10}N, and Cr{sub 0.90}Si{sub 0.10}N than for their binary counterparts TiN and CrN. Based on our results, we can conclude that TiN based systems outperform CrN based systems with respect to pure adhesion, while the Si-containing ternaries exhibit higher implantation barriers for Al and Fe atoms. The data obtained are important to understand the atomistic interaction of metal atoms with nitride-based materials, which is valid not just for machining operations but also for any combination such as interfaces between coatings and substrates or multilayer and phase arrangements themselves.
Wang, Yi-Siang; Yin, Chih-Chien; Chao, Sheng D., E-mail: sdchao@spring.iam.ntu.edu.tw [Institute of Applied Mechanics, National Taiwan University, Taipei 106, Taiwan (China)
2014-10-07
We perform an ab initio computational study of molecular complexes with the general formula CF{sub 3}X—B that involve one trifluorohalomethane CF{sub 3}X (X = Cl or Br) and one of a series of Lewis bases B in the gas phase. The Lewis bases are so chosen that they provide a range of electron-donating abilities for comparison. Based on the characteristics of their electron pairs, we consider the Lewis bases with a single n-pair (NH{sub 3} and PH{sub 3}), two n-pairs (H{sub 2}O and H{sub 2}S), two n-pairs with an unsaturated bond (H{sub 2}CO and H{sub 2}CS), and a single π-pair (C{sub 2}H{sub 4}) and two π-pairs (C{sub 2}H{sub 2}). The aim is to systematically investigate the influence of the electron pair characteristics and the central atom substitution effects on the geometries and energetics of the formed complexes. The counterpoise-corrected supermolecule MP2 and coupled-cluster single double with perturbative triple [CCSD(T)] levels of theory have been employed, together with a series of basis sets up to aug-cc-pVTZ. The angular and radial configurations, the binding energies, and the electrostatic potentials of the stable complexes have been compared and discussed as the Lewis base varies. For those complexes where halogen bonding plays a significant role, the calculated geometries and energetics are consistent with the σ-hole model. Upon formation of stable complexes, the C–X bond lengths shorten, while the C–X vibrational frequencies increase, thus rendering blueshifting halogen bonds. The central atom substitution usually enlarges the intermolecular bond distances while it reduces the net charge transfers, thus weakening the bond strengths. The analysis based on the σ-hole model is grossly reliable but requires suitable modifications incorporating the central atom substitution effects, in particular, when interaction components other than electrostatic contributions are involved.
Ab initio study of different structures of CaC: Magnetism, bonding, and lattice dynamics
Nourbakhsh, Zahra, E-mail: z.nourbakhsh@ph.iut.ac.ir; Hashemifar, S. Javad, E-mail: hashemifar@cc.iut.ac.ir; Akbarzadeh, Hadi
2013-12-05
Highlights: •Electronic structure and phonon calculations are performed on seven CaC structures. •The more covalent structures have lower energies and nonmagnetic ground state. •Ferromagnetism in the ionic phases comes from the sharp C p band at the Fermi level. •Tendency of C atoms for dimerization may lead to structural instabilities in CaC. •Nonmagnetic B33 CaC is stable in a wide range of temperatures and pressures. -- Abstract: On the basis of ab initio pseudopotential calculations, we study structural, magnetic, dynamical, and mechanical properties of the hypothetical CaC ionic compound in the rock-salt (RS), B2, zinc-blende (ZB), wurtzite (WZ), NiAs (NA), anti-NiAs (NA{sup ∗}), and CrB (B33) structures. It is argued that the ZB, WZ, NA, and RS structures are more ionic while the NA{sup ∗}, B2, and B33 structures are more covalent systems. As a result of that, the nonmagnetic B33–CaC is the energetically preferred system, while the more ionic structures prefer a ferromagnetic ground state with high Fermi level spin polarization. The observed ferromagnetism in the more ionic systems is attributed to the sharp partially filled p states of carbon atom in the system. In the framework of density functional perturbation theory, the phonon spectra of these systems are computed and the observed dynamical instabilities of the NA{sup ∗} and B2 structures are explained in terms of the covalent bonds between carbon atoms. The calculated Helmholtz and Enthalpy free energies indicate the highest stability of the B33 structure in a wide range of temperatures and pressures. Among the ferromagnetic structures, RS–CaC and ZB–CaC are reported, respectively, to be the most and the least metastable systems in various thermodynamics conditions. Several mechanical properties of the dynamically stable structures of CaC are determined from their phonon spectra.
Kim, B.
1990-10-01
This report discusses the following topics: molecular structure of NO{sub 3} radical studied by laser induced fluorescence; photodissociation and fluorescence spectroscopy of NO{sub 3} in molecular beam; vertical electronic spectrum of NO{sub 3}:{sup 2}A{prime}{sub 2}, {sup 2}E{double prime}({sup 2}A{sub 2}{sup 2}B{sub 1}), and {sup 2}E{prime} states; and Ab initio study of the vibrational spectra of NO{sub 3}.
An ab initio molecular dynamics study of the roaming mechanism of the H{sub 2}+HOC{sup +} reaction
Yu Huagen, E-mail: hgy@bnl.gov [Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973 (United States)
2011-08-01
We report here a direct ab initio molecular dynamics study of the p-/o-H{sub 2}+HOC{sup +} reaction on the basis of the accurate SAC-MP2 potential energy surface. The quasi-classical trajectory method was employed. This work largely focuses on the study of reaction mechanisms. A roaming mechanism was identified for this molecular ion-molecule reaction. The driving forces behind the roaming mechanism were thoroughly investigated by using a trajectory dynamics approach. In addition, the thermal rate coefficients of the H{sub 2}+HOC{sup +} reaction were calculated in the temperature range [25, 300] K and are in good agreement with experiments.
Ab initio study of the optical phonons in 1D antiferromagnet Ca2CuO3
Hoang, Nam Nhat; Nguyen, Thu Hang; Nguyen, Chau
2008-01-01
In the spin 1/2 antiferromagnetic Ca2CuO3 the optical excitation along 1D a Cu-O chain showed the enrichment of forbidden peaks that could not be explained by the group theory. We present the cluster-model ab initio study of these optical phonons based on the Hartree-Fock SCF calculation with the 3-21G basic set. The obtained results showed very good agreement with the observed data. The Cu-O resonances generally showed the lower shifts in Ca2CuO3 than in pure CuO and were primarily composed ...
Chemisorption of group-III metals on the Si(111) and Ge(111) surfaces: An ab initio study
Ricart, J M; Rubio Martínez, Jaime; Illas i Riera, Francesc
1990-01-01
Chemisorption of group-III metal adatoms on Si(111) and Ge(111) has been studied through the ab initio Hartree-Fock method including nonempirical pseudopotentials and using cluster models to simulate the surface. Three different high-symmetry sites (atop, eclipsed, and open) have been considered by using X4H9, X4H7, and X6H9 (X=Si,Ge) cluster models. In a first step, ideal surface geometries have been used. Metal-induced reconstruction upon chemisorption has also been taken into account. Equi...
Ab initio study of vibronic transitions between x2π and 12Σ+ electronic states of HCP+ ion
Stojanović Ljiljana
2013-01-01
Full Text Available The ground and low-lying excited doublet electronic states of the HCP+ ion were studied by means of multireference configuration interaction method. Vibronic energy levels of the X2Π state of Σ, Π, Δ, and Φ symmetry, up to the 2500 cm-1, have been calculated variationally, employing previously developed ab initio methods which take into account vibronic and spin-orbit interactions. Obtained vibronic wave functions were used to estimate transition moments between vibronic energy levels of the X2Π and 12Σ+ electronic states. Results were compared to available experimental and theoretical data. [Projekat Ministarstva nauke Republike Srbije, br. 172040
The temperature dependence of effective Waldmann--Snider cross sections determining relaxation and line broadening phenomena has been studied for p-H2 between 20 and 200 K. In particular, the rotational relaxation cross section and the relaxation cross sections of the rotational angular momentum vector and tensor polarizations and their respective fluxes have been calculated in an entirely ab initio treatment and close coupling formalism. As far as experimental results were available, quantitative agreement has been obtained. The rotational relaxation cross section has been proven to be quite sensitive to the potential anisotropy. Furthermore, the validity of some approximate relations between effective cross sections has been tested
Ab initio study of ferromagnetic La0.5Ba0.5CoO3
Umesh V Waghmare
2003-10-01
We study structure and magnetic properties of La0.5Ba0.5CoO3 (LBCO) using ab initio density functional theory (DFT) method based on pseudopotentials and a plane-wave basis. We find the cubic structure of LBCO is ferromagnetic and lower in energy than the nonmagnetic rhombohedral structure. Through the calculation of -point phonons of LBCO in the cubic structure, we determine its structural instabilities and find that they correspond to the structural phase transition accompanying a para-ferromagnetic transition observed recently.
Nagoaka, Shin-ichi; Sawada, Kouhei; Fukumoto, Youji [Ehime Univ., Matsuyama (Japan)] [and others
1992-08-06
This paper discusses using kinetic, spectroscopic, and ab initio studies of the prooxidant reaction of vitamin E derivatives to determine second-order rate constants for the reaction of six tocopheroxyl radicals with five alkyl hydroperoxides in benzene along with the first adiabatic ionization potentials of the alkyl hydroperoxides. These results suggest that charge transfer and proton tunneling play important roles in this prooxidant reaction, with proton tunneling taking place below the transition state and allowing the proton to cut a corner on the potential energy surface. 31 refs., 10 figs., 2 tabs.
Stacking stability of MoS2 bilayer: An ab initio study
The study of the stacking stability of bilayer MoS2 is essential since a bilayer has exhibited advantages over single layer MoS2 in many aspects for nanoelectronic applications. We explored the relative stability, optimal sliding path between different stacking orders of bilayer MoS2, and (especially) the effect of inter-layer stress, by combining first-principles density functional total energy calculations and the climbing-image nudge-elastic-band (CI-NEB) method. Among five typical stacking orders, which can be categorized into two kinds (I: AA, AB and II: AA', AB', A'B), we found that stacking orders with Mo and S superposing from both layers, such as AA' and AB, is more stable than the others. With smaller computational efforts than potential energy profile searching, we can study the effect of inter-layer stress on the stacking stability. Under isobaric condition, the sliding barrier increases by a few eV/(ucGPa) from AA' to AB', compared to 0.1 eV/(ucGPa) from AB to [AB]. Moreover, we found that interlayer compressive stress can help enhance the transport properties of AA'. This study can help understand why inter-layer stress by dielectric gating materials can be an effective means to improving MoS2 on nanoelectronic applications. (condensed matter: structural, mechanical, and thermal properties)
Guo, Xun; Zhang, Xitong; Zhao, Shijun; Huang, Qing; Xue, Jianming
2016-01-01
Density functional theory (DFT) calculation is employed to study the adsorption properties of Pb and Cu on recently synthesized two-dimensional materials MXenes, including Ti3C2, V2C1 and Ti2C1. The influence of surface decoration with functional groups such as H, OH and F have also been investigated. Most of these studied MXenes exhibit excellent capability to adsorb Pb and Cu, especially the adsorption capacity of Pb on Ti2C1 is as high as 2560 mg g(-1). Both the binding energies and the adsorption capacities are sensitive to the functional groups attached to the MXenes' surface. Ab initio molecular dynamics (ab-init MD) simulation confirms that Ti2C1 remains stable at room temperature after adsorbing Pb atoms. Our calculations imply that these newly emerging two-dimensional MXenes are promising candidates for wastewater treatment and ion separation. PMID:26602974
Giansiracusa, Marcus J; Vonci, Michele; Van den Heuvel, Willem; Gable, Robert W; Moubaraki, Boujemaa; Murray, Keith S; Yu, Dehong; Mole, Richard A; Soncini, Alessandro; Boskovic, Colette
2016-06-01
Optimization of literature synthetic procedures has afforded, in moderate yield, homogeneous and crystalline samples of the five analogues Na11[{RE(OH2)}3CO3(PW9O34)2] (1-RE; RE = Y, Tb, Dy, Ho, and Er). Phase-transfer methods have allowed isolation of the mixed salts (Et4N)9Na2[{RE(OH2)}3CO3(PW9O34)2] (2-RE; RE = Y and Er). The isostructural polyanions in these compounds are comprised of a triangular arrangement of trivalent rare-earth ions bridged by a μ3-carbonate ligand and sandwiched between two trilacunary Keggin {PW9O34} polyoxometalate ligands. Alternating-current (ac) magnetic susceptibility studies of 1-Dy, 1-Er, and 2-Er reveal the onset of frequency dependence for the out-of-phase susceptibility in the presence of an applied magnetic field at the lowest measured temperatures. Inelastic neutron scattering (INS) spectra of 1-Ho and 1-Er exhibit transitions between the lowest-lying crystal-field (CF) split states of the respective J = 8 and (15)/2 ground-state spin-orbit multiplets of the Ho(III) and Er(III) ions. Complementary ab initio calculations performed for these two analogues allow excellent reproduction of the experimental magnetic susceptibility and low-temperature magnetization data and are in reasonable agreement with the experimental INS data. The ab initio calculations reveal that the slight difference in coordination environments of the three Ln(III) ions in each complex gives rise to differences in the CF splitting that are not insignificant. This theoretical result is consistent with the observation of multiple relaxation processes by ac magnetic susceptibility and the broadness of the measured INS peaks. The ab initio calculations also indicate substantial mixing of the MJ contributions to the CF split energy levels of each Ln(III) ion. Calculations indicate that the CF ground states of the Ho(III) centers in 1-Ho are predominantly comprised of contributions from small MJ, while those of the Er(III) centers in 1-Er are predominantly
Ab initio valence calculations in chemistry
Cook, D B
1974-01-01
Ab Initio Valence Calculations in Chemistry describes the theory and practice of ab initio valence calculations in chemistry and applies the ideas to a specific example, linear BeH2. Topics covered include the Schrödinger equation and the orbital approximation to atomic orbitals; molecular orbital and valence bond methods; practical molecular wave functions; and molecular integrals. Open shell systems, molecular symmetry, and localized descriptions of electronic structure are also discussed. This book is comprised of 13 chapters and begins by introducing the reader to the use of the Schrödinge
A Comparative Study of Ab-Initio Thermal Conductivity Approaches: The Case of Cubic Boron Nitride
Mukhopadhyay, Saikat; Lindsay, Lucas; Broido, David; Stewart, Derek
2013-03-01
Given its high strength and large thermal conductivity, cubic boron nitride (cBN) provides an important complement to diamond films for heat spreading applications. However, cBN, in contrast to diamond, is a polar material with significant LO-TO splitting in the phonon dispersion. In this talk, we examine the lattice thermal conductivity of cBN using several approaches based on first principles calculations. These approaches include: (1) an analytic modified Callaway-Debye model that relies on parameters from ab-initio harmonic force constants, (2) a fully self-consistent calculation of the thermal conductivity that links an iterative solution of the phonon Boltzmann transport equation (BTE) with harmonic and anharmonic interatomic force constants. The force constants for the BTE are calculated using two approaches: density functional perturbation theory and a real-space supercell approach. We will compare the results from these approaches, highlight the role of normal phonon-phonon scattering, and also examine the impact of optical modes and LO-TO splitting. In addition, we will discuss how isotope scattering affects thermal conductivity and compare this to other boron nitride structures (hexagonal BN, BN sheets and BN nanotubes).
Ab initio study of the unusual thermal transport properties of boron arsenide and related materials
Broido, D. A.; Lindsay, L.; Reinecke, T. L.
2013-12-01
Recently, using a first principles approach, we predicted that zinc blende boron arsenide (BAs) will have an ultrahigh lattice thermal conductivity, κ, of over 2000 Wm-1K-1 at room temperature (RT), comparable to that of diamond. Here, we provide a detailed ab initio examination of phonon thermal transport in boron arsenide, contrasting its unconventional behavior with that of other related materials, including the zinc blende crystals boron nitride (BN), boron phosphide, boron antimonide, and gallium nitride (GaN). The unusual vibrational properties of BAs contribute to its weak phonon-phonon scattering and phonon-isotope scattering, which are responsible for its exceptionally high κ. The thermal conductivity of BAs has contributions from phonons with anomalously large mean free paths (˜2 μm), two to three times those of diamond and BN. This makes κ in BAs sensitive to phonon scattering from crystal boundaries. An order of magnitude smaller RT thermal conductivity in a similar material, zinc blende GaN, is connected to more separated acoustic phonon branches, larger anharmonic force constants, and a large isotope mixture on the heavy rather than the light constituent atom. The striking difference in κ for BAs and GaN demonstrates the importance of using a microscopic first principles thermal transport approach for calculating κ. BAs also has an advantageous RT coefficient of thermal expansion, which, combined with the high κ value, suggests that it is a promising material for use in thermal management applications.
Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations
Ab initio study of Cr interactions with point defects in bcc Fe
Full text of publication follows. Ferritic martensitic steels are candidate structural materials for fast neutron reactors, and in particular high-Cr reduced-activation steels. In Fe-Cr alloys, Cr plays a major role in the radiation-induced evolution of the mechanical properties. Using ab initio calculations based on density functional theory, the properties of Cr in α-Fe have been investigated. The intrinsic point defect formation energies were found to be larger in model bcc Cr as compared to those in ferromagnetic bcc Fe. The interactions of Cr with point defects (vacancy and self interstitials) have been characterised. Single Cr atoms interact weakly with vacancies but significantly with self-interstitial atoms. Mixed interstitials of any interstitial symmetry are bound. Configurations where two Cr atoms are in nearest neighbour position are generally unfavourable in bcc Fe except when they are a part of a interstitial complex. Mixed interstitials do not have as strong directional stability as pure Fe interstitials have. The effects on the results using the atom description scheme of either the ultrasoft pseudo-potential (USPP) or the projector augmented wave (PAW) formalisms are connected to the differences in local magnetic moments that the two methods predict. As expected for the Fe-Cr system, the results obtained using the PAW method are more reliable than the ones obtained with USPP. (authors)
Laser spectroscopy and ab initio studies of metal-containing free radicals
Greetham, G M
2000-01-01
strontium-containing free radical is reported, that of SrCCH. This new excited electronic state is accessed by the orbitally-forbidden B-tilde' sup 2 DELTA-X-tilde sup 2 SIGMA sup + transition. Spin-orbit and vibrational structure have been seen in spectra of SrCCH and SrCCD and confirmed the assignment. Finally, observation of a new transition in an unidentified gallium-containing molecule is reported. Two progressions corresponding to two different vibrational modes of the molecule are seen in the spectrum. Potential spectral carriers, including Ga sub x clusters and other gallium-containing molecules formed by reaction with impurities, are discussed in an attempt to explain the observed spectrum. This work describes the use of laser spectroscopy and ab initio calculations in the investigation of several new electronic transitions in metal-containing free radicals. These free radicals were prepared in a supersonic jet by laser ablation of solid metal samples in the presence of appropriate precursor molecule...
Ab initio studies of transition metal complexes and related electron transfer properties
Ab initio electronic structure calculations have been carried out for various aquo and ammine complexes of Fe, Co, and Ru in their 2+ and 3+ oxidation states. The results of these calculations are used as assessing a variety of factors controlling electron transfer kinetics including the charge-state dependence of various inner shell geometrical parameters and associated vibrational frequencies (metal-ligand and intra-ligand and intra-ligand stretching modes, and librational modes), and the dependence of electron transfer matrix elements on the nature of the ligand and the metal orbital type (/sup t/2/sub g/ vs. e/sub g/). The charge-state dependence of the OH bond lengths in hexa-aquo ions is predicted to yield H/D isotope effects (i.e., fractionation between bulk and first shell water), which should be detectable by neutron scattering experiments on appropriate aqueous solutions. The relationship between these thermodynamic isotope effects, which depend critically on strong OH triple bond O hydrogen bonding between first and second-shell water molecules, and kinetic H/D isotope effects in electron exchange involving hexa-aquo ions will be discussed
Ab-initio study of the dielectric response of high-permittivity perovskites for energy storage
Many of materials based on transition metals have a wide range of applications, such as the storage of energy, due to their peculiar properties (high-dielectric constants, ferro-electricity,...). The knowledge of their bulk properties is essential in designing targeted devices with high performance. For instance, ABO3 perovskites are peculiarly interesting for their atomic structural flexibility, allowing high number of atoms substitution and giving them specific chemical and electrical properties compared to the pure compounds. In this context, first principles calculations can be useful to understand the structural and electronic properties of these materials. The pressure-induced giant dielectric anomaly of ABO3 perovskites has been investigated at the ab initio level. Its mechanism has been analyzed in terms of thermodynamic phase stability, structural and phonon contributions and Born effective charges. It is shown that the IR-active soft phonon is responsible for the anomaly. This mode always involves a displacement and a deformation of the oxygen octahedra, while the roles of A and B ions vary among the materials and between high- and low-pressure phase transitions. A sharp increase in the phonon amplitude near the phase transition gives rise to the dielectric anomaly. The use of hybrid functionals is required for agreement with experimental data. The calculations show that the dielectric anomaly in the pressure-induced phase transitions of these perovskites is a property of the bulk material. (author)
Ab initio Mechanism Study on the Reaction of Chlorine Atom with Formic Acid
于海涛; 付宏刚; 等
2003-01-01
The potential energy surface(PES) for the reaction of Cl atom with HCOOH is predicted using ab initio molecular orbital calculation methods at UQCIDS(T,full)6-311++G(3df,2p)//UMP2(full)/6-311+G(d,P) level of theory with zero-point vibrational energy (ZPVE) correction.The calculated results show that the reaction mechanism of Cl atom with formic acid is a C-site hydrogen abstraction reaction from cis-HOC(H)O molecule by Cl atom with a 3.73kJ/mol reaction barrier height,leading to the formation of cis-HOCO radical which will reacts with Cl atom or other molecules in such a reaction system.Because the reaction barrier height of O-site hydrogen abstraction reaction from cis-HOC(H)O molecule by Cl atom which leads to the formation of HCO2 radical is 67.95kJ/mol,it is a secondary reaction channel in experiment,This is in good agreement with the prediction based on the previous experiments.
An ab initio study of magneto-electric coupling of YMnO3
This paper proposes the direct calculation of the microscopic contributions to the magneto-electric coupling, using ab initio methods. The electrostrictive and the Dzyaloshinskii–Moriya contributions were evaluated individually. For this purpose a specific method was designed, combining density functional theory calculations and embedded fragment, explicitly correlated, quantum chemical calculations. This method allowed us to calculate the evolution of the magnetic couplings as a function of an applied electric field. We found that in YMnO3 the Dzyaloshinskii–Moriya contribution to the magneto-electric effect is three orders of magnitude weaker than the electrostrictive contribution. Strictive effects are thus dominant in the magnetic exchange evolution under an applied electric field, and by extension in the magneto-electric effect. These effects however, remain quite small, and the modifications of the magnetic excitations under an applied electric field will be difficult to observe experimentally. Another important conclusion is that it can be shown that the linear magneto-electric tensor is null due to the inter-layer symmetry operations. (paper)
Ab initio energetic study of oxide ceramics with rare-earth elements
WU Bo; Matvei Zinkevich; WANG Chong; Fritz Aldinger
2006-01-01
Ab initio energetic calculations based on the density functional theory (DFT) and the projector augmented wave method (PAW) for determining the polymorphisms of lanthanide sesquioxides Ln2O3 (where Ln = rare-earth element. Y,and Sc), LnMO3 perovskites (where M = Al and Ga), and Ln2B2O7 pyrochlores (where B = Ti, Zr, and Hf) were reported. The relative lattice stabilities agreed well with the critically assessed results or the experimental results except the C-type Ln2O3 with a cubic structure, for which the calculated total energies were considerably more negative. With the increase of the Ln3+-cation radius, the polymorphic structures showed a degenerative tendency. The tendencies and quantities of the enthalpies of formation of the ternary oxide ceramics synthesized from their constituent binary oxides reasonably agreed with the available experimental results, and valuable thermodynamic properties were afforded to the compound, for which no experimental data is available. The enthalpies of formation of both perovskites and pyrochlores tend to become more negative with the increase of the Ln3+-cation radius.
Switchable magnetic moment in cobalt-doped graphene bilayer on Cu(111): An ab initio study
Souza, Everson S.; Scopel, Wanderlã L.; Miwa, R. H.
2016-06-01
In this work, we have performed an ab initio theoretical investigation of substitutional cobalt atoms in the graphene bilayer supported on the Cu(111) surface (Co/GBL/Cu). Initially, we examined the separated systems, namely, graphene bilayer adsorbed on Cu(111) (GBL/Cu) and a free standing Co-doped GBL (Co/GBL). In the former system, the GBL becomes n -type doped, where we map the net electronic charge density distribution along the GBL-Cu(111) interface. The substitutional Co atom in Co/GBL lies between the graphene layers, and present a net magnetic moment mostly due to the unpaired Co-3 dz2 electrons. In Co/GBL/Cu, we found that the Cu(111) substrate rules (i) the energetic stability, and (ii) the magnetic properties of substitutional Co atoms in the graphene bilayer. In (i), the substitutional Co atom becomes energetically more stable lying on the GBL surface, and in (ii), the magnetic moment of Co/GBL has been quenched due to the Cu(111) → Co/GBL electronic charge transfer. We verify that such a charge transfer can be tuned upon the application of an external electric field, and thus mediated by a suitable change on the electronic occupation of the Co-dz2 orbitals, we found a way to switch-on and -off the magnetization of the Co-doped GBL adsorbed on the Cu(111) surface.
Liu, Shi-Yu, E-mail: buaasyliu@gmail.com [College of Physics and Electronic Information Science, Tianjin Normal University, Tianjin 300387 (China); Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong (China); Liu, Shiyang [Institute of Information Optics, Zhejiang Normal University, Jinhua, Zhejiang 321004 (China); Li, De-Jun [College of Physics and Electronic Information Science, Tianjin Normal University, Tianjin 300387 (China); Wang, Sanwu, E-mail: sanwu-wang@utulsa.edu [Department of Physics and Engineering Physics, The University of Tulsa, Tulsa, Oklahoma 74104 (United States); Guo, Jing; Shen, Yaogen, E-mail: meshen@cityu.edu.hk [Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong (China)
2015-02-14
Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.
Anomalous structural dynamics in liquid Al80Cu20: An ab initio molecular dynamics study
In this work, the structural dynamics of liquid Al80Cu20 is systematically investigated in terms of the evolution of its atomic structure, diffusivity, viscosity and fragility through ab initio molecular dynamics simulations. In addition to using pair correlation functions and coordination numbers, the various local ordered clusters are characterized comprehensively by Honeycutt-Anderson bond pairs and Voronoi polyhedra. Compared to the self diffusivity of pure liquid Cu, the tracer diffusion coefficients of Cu in liquid Al80Cu20 are increased, in agreement with the results measured by quasielastic neutron-scattering (QENS). Although the interdiffusion coefficients predicted by Darken’s equation match well to those obtained from the viscosity measurements via the Stokes-Einstein relation, they are smaller than those measured by QENS or X-ray radiography, indicative of an anomalous nature of the structural dynamics, dominated by the local ordered clusters in liquid Al80Cu20. Furthermore, Vogel–Fulcher–Tammann fitting results indicate that the liquid Al80Cu20 can be classified into a strong liquid. The deformation electron density shows that the intrinsic tetrahedral-type bonds in FCC Al and Cu are transformed into an amorphous type in liquid Al80Cu20. The present work provides insights into the understanding of structural dynamics and the kinetic properties of such metallic melts
Ab initio study of intrinsic defects and diffusion behaviors in solid molecular hydrogens
Sun, Qingqiang; Ye, Yingting; Yang, Tianle; Yang, Li; Peng, Shuming; Long, Xinggui; Zhou, Xiaosong; Zu, Xiaotao; Du, Jincheng
2015-12-01
The behaviors of intrinsic defects in solid molecular hydrogens (H2) were investigated using ab initio calculations based on density functional theory. The results show that the formation energy of a vacancy is dependent on molecule orientation in disordered hexagonal close-packed (hcp) H2 crystals, but independent of molecular orientation in face-centered cubic-Pa3 H2. Furthermore, H2 molecules generally prefer to occupy the basal octahedral sites in hcp and octahedral sites in Pa3 structures. The formation energies of an interstitial H2 depend sensitively on the volume of interstitial sites, and also on near spatial distributions of molecular axes of a H2 in hcp H2 crystals. The strong force field introduced by an interstitial H2 might induce the rotation of molecular axes and reduce the formation energy of the interstitial H2. The migration barrier energy of a vacancy in Pa3 is larger than the average in hcp structures. However, the H2 molecules prefer to jump though vacancies in the two structures of solid H2 than the interstitial migrations.
Ab initio Study on Structures and Isomerization of Magnesium Fluorosilylenoid H2SiFMgF
Yi-jian Zhang; Mei-jiang Li; Guo-qiao Lai; Da-cheng Feng; Sheng-yu Feng
2008-01-01
The structures and isomerization of magnesium fluorosilylenoid H2SiFMgF were investigated by ab initio molecular orbital theory. Four equilibrium structures and three isomeric transition states were located and fully optimized at the B3LYP/6-31G(d,p) and G3MP2B3 levels, respectively. Based on the B3LYP/6-31G(d,p) optimized geometries, harmonic frequencies of various structures were obtained and 29Si chemical shifts were calculated. The solvent effects were investigated by means of the polarizable continuum model using THF as a solvent at B3LYP/6-31G(d,p) level. Isomerization paths for isomers were confirmed by in-trinsic reaction coordinate calculations. The calculated results show that tetrahedral structure has the lowest energy and is the most stable; tetrahedral, three-membered ring, and p-complex structures are suggested to be the experimentally detectable ones; and σ-complex structure has the highest energy and will not exist.
Ferromagnetic ordering of Cr and Fe doped p-type diamond: An ab initio study
Ferromagnetic ordering of transition metal dopants in semiconductors holds the prospect of combining the capabilities of semiconductors and magnetic systems in single hybrid devices for spintronic applications. Various semiconductors have so far been considered for spintronic applications, but low Curie temperatures have hindered room temperature applications. We report ab initio DFT calculations on the stability and magnetic properties of Fe and Cr impurities in diamond, and show that their ground state magnetic ordering and stabilization energies depend strongly on the charge state and type of co-doping. We predict that divacancy Cr+2 and substitutional Fe+1 order ferromagnetically in p-type diamond, with magnetic stabilization energies (and magnetic moment per impurity ion) of 16.9 meV (2.5 μB) and 33.3 meV (1.0 μB), respectively. These magnetic stabilization energies are much larger than what has been achieved in other semiconductors at comparable impurity concentrations, including the archetypal dilute magnetic semiconductor GaAs:Mn. In addition, substitutional Fe+1 exhibits a strong half-metallic character, with the Fermi level crossing bands in only the spin down channel. These results, combined with diamond’s extreme properties, demonstrate that Cr or Fe dopedp-type diamond may successfully be considered in the search for room temperature spintronic materials
Bakaev, A. [SCK-CEN, Nuclear Materials Science Institute, Boeretang 200, Mol B2400 (Belgium); Center for Molecular Modeling, Department of Physics and Astronomy, Ghent University, Technologiepark 903, 9052 Zwijnaarde (Belgium); Department of Experimental Nuclear Physics K-89, Faculty of Physics and Mechanics, St. Petersburg State Polytechnical University, 29 Polytekhnicheskaya str., 195251 St. Petersburg (Russian Federation); Terentyev, D., E-mail: dterenty@sckcen.be [SCK-CEN, Nuclear Materials Science Institute, Boeretang 200, Mol B2400 (Belgium); Bonny, G. [SCK-CEN, Nuclear Materials Science Institute, Boeretang 200, Mol B2400 (Belgium); Klaver, T.P.C. [Department of Materials Science and Engineering, Faculty of 3mE, TU Delft, Mekelweg 2, 2628 CD Delft (Netherlands); Olsson, P. [Department of Neutron Research, Angstrom Laboratory, Uppsala University, Box 525, SE-75120 Uppsala (Sweden); Van Neck, D. [Center for Molecular Modeling, Department of Physics and Astronomy, Ghent University, Technologiepark 903, 9052 Zwijnaarde (Belgium)
2014-01-15
Highlights: • The interaction of Mo, W, Nb, Ta, V, Mn, Si with point and extended defects is characterized. • Mn and Si exhibit peculiar interaction with both point and extended lattice defects. • The results for substitutional atoms of the refractory metals well follow one specific trend. -- Abstract: Basic properties of minor alloying elements, namely Mo, W, Nb, Ta, V, Mn, Si entering the conventional and reduced-activation structural Fe–(9–12)Cr steels have been analyzed using ab initio calculations. The electronic structure calculations were applied to study the interaction of minor alloying elements with a number of important and well defined lattice structures, such as point defects, the 1/2〈1 1 1〉 screw dislocation core, high angle symmetric grain boundaries and free surfaces. The studied elements were classified according to their similarities and discrepancies regarding the interaction with the above mentioned defects. The refractory alloying elements are found to follow the same trend whereas Mn and Si exhibit peculiar behavior with respect to the interaction with both point and extended lattice defects. The obtained results are discussed and compared with previously published ab initio and available experimental data.
Ab initio theoretical comparative study of magnetic coupling in KNiF3sand K2 NiF4s
de P. R. Moreira, Ibério; Illas, Francesc
1997-02-01
The origin of magnetic coupling in KNiF3 and K2 NiF4 is studied by means of an ab initio cluster model approach. By a detailed study of the mapping between eigenstates of the exact nonrelativistic and spin model Hamiltonians it is possible to obtain the magnetic coupling constant J and to compare ab initio cluster-model values with those resulting from ab initio periodic Hartree-Fock calculations. This comparison shows that J is strongly determined by two-body interactions; this is a surprising and unexpected result. The importance of the ligands surrounding the basic metal-ligand-metal interacting unit is reexamined by using two different partitions and the constrained space orbital variation method of analysis. This decomposition enables us to show that this effect is basically environmental. Finally, dynamical electronic correlation effects have found to be critical in determining the final value of the magnetic coupling constant.
A theoretical-spectroscopy, ab-initio-based study of the electronic ground state of 121SbH3
Yurchenko, Sergei N.; Carvajal Zaera, Miguel; Yachmenev, Andrey; Thiel, Walter; Jensen, Per
2010-01-01
For the stibine isotopologue (SbH3)-Sb-121, we report improved theoretical calculations of the vibrational energies below 8000 cm- and simulations of the rovibrational spectrum in the 0-8000 cm(-1) region. The calculations are based on a refined ab initio potential energy surface and on a new dipole moment surface obtained at the coupled cluster CCSD(T) level. The theoretical results are compared with the available experimental data in order to validate the ab initio surfaces and the TROVE co...
Rotational study of the CH4-CO complex: Millimeter-wave measurements and ab initio calculations.
Surin, L A; Tarabukin, I V; Panfilov, V A; Schlemmer, S; Kalugina, Y N; Faure, A; Rist, C; van der Avoird, A
2015-10-21
The rotational spectrum of the van der Waals complex CH4-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110-145 GHz. Newly observed and assigned transitions belong to the K = 2-1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2-1 and K = 0-1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4-CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4-CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm(-1). The bound rovibrational levels of the CH4-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm(-1) for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4-CO, respectively. PMID:26493903
Ab initio studies on the spin-forbidden cooling transitions of the LiRb molecule.
You, Yang; Yang, Chuan-Lu; Zhang, Qing-Qing; Wang, Mei-Shan; Ma, Xiao-Guang; Liu, Wen-Wang
2016-07-20
The spin-forbidden cooling of the LiRb molecule is investigated based on ab initio quantum chemistry calculations. The multireference configuration interaction method is used to generate the potential energy curves (PECs) of the ground state X(1)Σ(+) and the low-lying excited states a(3)Σ(+), B(1)Π, and b(3)Π. The spin-orbit coupling effects for the PECs and the transition dipole moments (TDMs) between the X(1)Σ(+), b(3)Π and a(3)Σ(+) states are also calculated. The analytical functions for the PECs are deduced. The rovibrational energy levels, the spectroscopic parameters and the Franck-Condon factors (FCF) are determined by solving the Schrödinger equation of nuclear movement with the obtained analytical functions. The b(3)Π0 ↔ X(1)Σ(+) and b(3)Π1 ↔ X(1)Σ(+) transitions have highly diagonal distributed FCFs and non-zero TDMs, demonstrating that the LiRb molecule could be a very promising candidate for laser cooling. Therefore, a three-cycle laser cooling scheme for the molecule has been proposed based on these two spin-forbidden transitions. Using the radiative lifetime and linewidth calculated from the obtained TDM functions, we present further analysis of the cooling of LiRb and the corresponding KRb molecule. The transition b(3)Π0 ↔ X(1)Σ(+) is found to be a practical transition to cool the LiRb molecule, and a sub-microkelvin cool temperature could be reached for the KRb molecule using a similar laser cooling scheme. PMID:27388722
Ab initio simulations have been performed to study the structure, energetics and stability of several plausible stacking sequences in graphite. These calculations suggest that in addition to the standard structures, graphite can also exist in AA-simple hexagonal, AB-orthorhombic and ABC-hexagonal type stacking. The free energy difference between these structures is very small (∼1 meV/atom), and hence all the structures can coexist from purely energetic considerations. Calculated x-ray diffraction patterns are similar to those of the standard structures for 2θ ⩽ 70°. Shear elastic constant C44 is negative in AA-simple hexagonal, AB-orthorhombic and ABC-hexagonal structures, suggesting that these structures are mechanically unstable. Phonon dispersions show that the frequencies of some modes along the Γ–A direction in the Brillouin zone are imaginary in all of the new structures, implying that these structures are dynamically unstable. Incorporation of zero point vibrational energy via the quasi-harmonic approximation does not result in the restoration of dynamical stability. Potential energy surfaces for the unstable normal modes are seen to have the topography of a potential hill for all the new structures, confirming that all of the new structures are inherently unstable. The fact that the potential energy surface is not in the form of a double well implies that the structures are linearly as well as globally unstable. (paper)
Stavretis, Shelby E; Atanasov, Mihail; Podlesnyak, Andrey A; Hunter, Seth C; Neese, Frank; Xue, Zi-Ling
2015-10-19
Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe(TPP)X] (X = F, Br, I; H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are D = 4.49(9) cm⁻¹ for tetragonal polycrystalline [Fe(TPP)F], and D = 8.8(2) cm⁻¹, E = 0.1(2) cm⁻¹ and D = 13.4(6) cm⁻¹, E = 0.3(6) cm⁻¹ for monoclinic polycrystalline [Fe(TPP)Br] and [Fe(TPP)I], respectively. Along with our recent report of the ZFS value of D = 6.33(8) cm⁻¹ for tetragonal polycrystalline [Fe(TPP)Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe(TPP)X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting D of the ⁶A₁ ground state. D was calculated from wave functions of the electronic multiplets spanned by the d⁵ configuration of Fe(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of D from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the S = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX₆³⁻ complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of D and decrease of the π- and σ-antibonding energies e(λ)(X) (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the
Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe(TPP)X] (X = F, Br, I; H2TPP = tetraphenylporphyrin) are determined by inelastic neutron scattering (INS). The ZFS values are D = 4.49(9) cm-1 for tetragonal polycrystalline [Fe(TPP)F], and D = 8.8(2) cm-1, E = 0.1(2) cm-1 and D = 13.4(6) cm-1, E = 0.3(6) cm-1 for monoclinic polycrystalline [Fe(TPP)Br] and [Fe(TPP)I], respectively. Along with our recent report of the ZFS value of D = 6.33(8) cm-1 for tetragonal polycrystalline [Fe(TPP)Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe(TPP)X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting D of the 6A1 ground state. D was calculated from wave functions of the electronic multiplets spanned by the d5 configuration of Fe(III) along with spin-orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of D from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the S = 3/2 excited states on ligand field theory was used to characterize the σ- and Π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX63- complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of D and decrease of the Π- and σ-antibonding energies e(λ)(X) (λ = σ, Π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. Furthermore, D
Silvestrelli, P.-L.; Alavi, A.; Parrinello, M.; Frenkel, D.
1997-01-01
The method of ab initio molecular dynamics, based on finite-temperature density-functional theory, is used to simulate laser heating of crystalline silicon. We found that a high concentration of excited electrons dramatically weakens the covalent bonding. As a result the system undergoes a melting t
An ab initio study on the ground and low-lying doublet electronic states of linear C2As
Graphical abstract: Here we report the results of ab initio calculations of the geometry, adiabatic excitation energies and electric dipole moments of the low-lying doublet electronic states of linear C2As. We also discuss some peculiar details of the vibronic and spin-orbit structure in the X∼2Π and 12Δ states, being the subject of our previous paper. The results for transition moments between the vibronic energy levels within the ground electronic state are presented. Research highlights: → The present study represents a continuation of our theoretical work on spectroscopy of C2As. → We determine the equilibrium geometry and adiabatic transition energies of the low-lying doublet excited states. → We presented the vibronic transitions within the ground X∼2Π electronic state. - Abstract: The present study is closely related to a recently published paper on the X∼2Π-12Δ spectral system of C2As [S. Jerosimic, Lj. Stojanovic, M. Peric, J. Chem. Phys. 133 (2010) 024307]. Here we report the results of ab initio calculations of the geometry, adiabatic excitation energies and electric dipole moments of the low-lying doublet electronic states of linear C2As. We also discuss some peculiar details of the vibronic and spin-orbit structure in the X∼2Π and 12Δ states, being the subject of our previous paper. The results for transition moments between the vibronic energy levels within the ground electronic state are presented. The aim of the present study is to motivate new experimental studies on the title molecule and to facilitate the interpretation of the results of these investigations.
Ab initio study of the lattice stability of β-SiC under intense laser irradiation
Shen, Yanhong; Gao, Tao, E-mail: gaotao@scu.edu.cn
2015-10-05
Highlights: • The band gap of β-SiC vanishes and its metallic character is presented when T{sub e} > 6 eV. • The TA modes of β-SiC are found to be negative T{sub e} = 3.39 eV. • The LO–TO splitting degree of β-SiC at Γ point begin to decline as T{sub e} > 4.5 eV. • The ionic strength of β-SiC is related to laser radiation intensity. - Abstract: We have performed ab initio calculation of electronic properties, lattice-dynamical properties, charge density difference and charge density of β-SiC at different electronic temperatures (T{sub e}) using local density approximation (LDA) pseudopotential method within the density functional perturbation theory (DFPT). The results of electronic density of state display that β-SiC is still semiconductor with band gap of 1.51 eV at T{sub e} = 0 eV. But, beyond a temperature of 6 eV, the band gap of β-SiC vanishes and its metallic character is presented. The calculated phonon frequencies of β-SiC at T{sub e} = 0 eV show a good agreement with the experimental values and other calculations. However, when β-SiC undergoes a sharp increase of its electronic temperature, the phonon frequencies of β-SiC have a significant softening. The transverse acoustic modes of β-SiC are found to be negative T{sub e} = 3.39 eV which lead to the lattice instability. Moreover, the LO–TO splitting degree of β-SiC at Γ point increases at first and then reduces as T{sub e} is raised, the turning point is at T{sub e} = 4.5 eV. By using CUT3D, the results of the charge density difference and charge density of β-SiC indicate that when radiation intensity is only strong enough (e.g. T{sub e} > 4.5 eV), it will make the ionic strength of β-SiC weaken. Otherwise, when radiation intensity is not very high (e.g. 0–4.5 eV), the ionic strength of β-SiC will increase with the rise of T{sub e}.
For liquid-sodium-cooled fast nuclear reactor systems, it is crucial to understand the behavior of lanthanides and other potential fission products in liquid sodium or other liquid metal solutions such as liquid cesium-sodium. In this study, we focus on lanthanide behavior in liquid sodium. Using ab initio molecular dynamics, we found that the solubility of cerium in liquid sodium at 1000 K was less than 0.78 at. %, and the diffusion coefficient of cerium in liquid sodium was calculated to be 5.57 × 10−9 m2/s. Furthermore, it was found that cerium in small amounts may significantly alter the heat capacity of the liquid sodium system. Our results are consistent with the experimental results for similar materials under similar conditions
Samin, Adib; Li, Xiang; Zhang, Jinsuo [Nuclear Engineering Program, Department of Mechanical and Aerospace Engineering, The Ohio State University, 201 W 19th Avenue, Columbus, Ohio 43210 (United States); Mariani, R. D. [Idaho National Laboratory, Materials and Fuels Complex, Idaho Falls, Idaho 83415 (United States); Unal, Cetin [Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, New Mexico 87545 (United States)
2015-12-21
For liquid-sodium-cooled fast nuclear reactor systems, it is crucial to understand the behavior of lanthanides and other potential fission products in liquid sodium or other liquid metal solutions such as liquid cesium-sodium. In this study, we focus on lanthanide behavior in liquid sodium. Using ab initio molecular dynamics, we found that the solubility of cerium in liquid sodium at 1000 K was less than 0.78 at. %, and the diffusion coefficient of cerium in liquid sodium was calculated to be 5.57 × 10{sup −9} m{sup 2}/s. Furthermore, it was found that cerium in small amounts may significantly alter the heat capacity of the liquid sodium system. Our results are consistent with the experimental results for similar materials under similar conditions.
Samin, Adib; Li, Xiang; Zhang, Jinsuo; Mariani, R. D.; Unal, Cetin
2015-12-01
For liquid-sodium-cooled fast nuclear reactor systems, it is crucial to understand the behavior of lanthanides and other potential fission products in liquid sodium or other liquid metal solutions such as liquid cesium-sodium. In this study, we focus on lanthanide behavior in liquid sodium. Using ab initio molecular dynamics, we found that the solubility of cerium in liquid sodium at 1000 K was less than 0.78 at. %, and the diffusion coefficient of cerium in liquid sodium was calculated to be 5.57 × 10-9 m2/s. Furthermore, it was found that cerium in small amounts may significantly alter the heat capacity of the liquid sodium system. Our results are consistent with the experimental results for similar materials under similar conditions.
Grassi, A; Angilella, G G N; March, N H; Pucci, R
2012-01-01
Fingerprints of antiaromaticity in the negative ion (Li_3Al_4)^-, this species being realizable via a laser vaporization technique, are revealed by means of an ab initio quantum-chemical investigation. First, the ground-state equilibrium geometry of this ion is predicted. Also, the characteristics of the HOMO are studied, both for the square and the rectangular Al_4 geometry in two low-lying isomers of the negative ion. There is no particular sensitivity to the change in geometry of the Al_4 configuration. Therefore, we have calculated theoretically chemical shifts, which contain remarkable fingerprints of antiaromaticity. As to future directions, some comments are added in relation to the Shannon entropy.
Calderín, L; González, L E; González, D J
2009-05-21
We report a study on several static, dynamic, and electronic properties of liquid Hg at room temperature. We have performed ab initio molecular dynamics simulations using Kohn-Sham density functional theory combined with a nonlocal ultrasoft pseudopotential. The calculated static structure shows good agreement with the available experimental data. We present results for the single-particle dynamics, and recent experimental data are analyzed. The calculated dynamic structure factors S(q,omega) fairly agree with their experimental counterparts as measured by inelastic x-ray (and neutron) scattering experiments. The dispersion relation exhibits a positive dispersion, which however is not so marked as suggested by the experiment; moreover, its slope at the long-wavelength limit provides a good estimate of the experimental sound velocity. We have also analyzed the dynamical processes behind the S(q,omega) in terms of a model including a relaxation mechanism with both fast and slow characteristic time scales. PMID:19466841
Ab initio mass tensor molecular dynamics
Tsuchida, Eiji
2010-01-01
Mass tensor molecular dynamics was first introduced by Bennett [J. Comput. Phys. 19, 267 (1975)] for efficient sampling of phase space through the use of generalized atomic masses. Here, we show how to apply this method to ab initio molecular dynamics simulations with minimal computational overhead. Test calculations on liquid water show a threefold reduction in computational effort without making the fixed geometry approximation. We also present a simple recipe for estimating the optimal ato...
Thiessen, P. A.; Treder, H.-J.
Jedes initium wird durch experimenta crucis zum eventus. Jedes theoretisch interpretierbare ex-eventu-Resultat führt auf ein neues Initium. Gerade dies ist die gemeinsame Aussage von Atomistik, Quantenmechanik und Relativitätstheorie.Translated AbstractAb initio vel ex eventu. IIEvery initium becomes an eventus by experimenta crucis. Every theoretically interpretable ex-eventu result leads to a new initium. Right this is the joint assertion of atomism, quantum mechanics, and relativity.
Highly scalable Ab initio genomic motif identification
Marchand, Benoît
2011-01-01
We present results of scaling an ab initio motif family identification system, Dragon Motif Finder (DMF), to 65,536 processor cores of IBM Blue Gene/P. DMF seeks groups of mutually similar polynucleotide patterns within a set of genomic sequences and builds various motif families from them. Such information is of relevance to many problems in life sciences. Prior attempts to scale such ab initio motif-finding algorithms achieved limited success. We solve the scalability issues using a combination of mixed-mode MPI-OpenMP parallel programming, master-slave work assignment, multi-level workload distribution, multi-level MPI collectives, and serial optimizations. While the scalability of our algorithm was excellent (94% parallel efficiency on 65,536 cores relative to 256 cores on a modest-size problem), the final speedup with respect to the original serial code exceeded 250,000 when serial optimizations are included. This enabled us to carry out many large-scale ab initio motiffinding simulations in a few hours while the original serial code would have needed decades of execution time. Copyright 2011 ACM.
Castellano, O; Bermúdez, Y; Giffard, M; Mabon, G; Cubillan, N; Sylla, M; Nguyen-Phu, X; Hinchliffe, A; Soscún, H
2005-11-17
The geometries and the static dipole (hyper)polarizabilities (alpha, beta, gamma) of a series of aromatic anions were investigated at the ab initio (HF, MP2, and MP4) and density functional theory DFT (B3LYP) levels of theory. The anions chosen for the present study are the benzenethiolate (Ph-S-), benzenecarboxylate (Ph-CO2-), benzenesulfinate (Ph-SO2-), benzenesulfonate (Ph-SO3-), and 1,3-benzenedicarboxylate (1,3-Ph-(CO2)2(2-)). For benzenethiolate anion, additional alpha, beta, and gamma calculations were performed at the coupled cluster CCSD level with MP2 optimized geometries. The standard diffuse and polarized 6-31+G(d,p) basis set was employed in conjunction to the ab initio and DFT methods. Additional HF calculations were performed with the 6-311++G(3d,3p) basis set for all the anions. The correlated electric properties were evaluated numerically within the formalism of finite field. The optimized geometries were analyzed in terms of the few reports about the phenolate and sulfonate ions. The results show that electron correlation effects on the polarizabilities are very important in all the anion series. Was found that Ph-SO2- is highly polarizable in terms of alpha and beta, and the Ph-S- is the highest second hyperpolarizable in the series. The results of alpha were rationalized in terms of the analysis of the polarization of charge based in Mulliken atomic population and the structural features of the optimized geometries of anions, whereas the large differences in the beta and gamma values in the series were respectively interpreted in terms of the bond length alternation BLA and the separation of charge in the aromatic ring by effects of the substitution. These results allowed us to suggest the benzenesulfinate and benzenethiolate anions as promising candidates that should be incorporated in ionic materials for second and third-order nonlinear optical devices. PMID:16833334
An experimental and ab initio study of the electronic spectrum of the jet-cooled F2BO free radical
We have studied the B~2A1–X~2B2 laser-induced fluorescence (LIF) spectrum of the jet-cooled F2BO radical for the first time. The transition consists of a strong 000 band at 446.5 nm and eight weak sequence bands to shorter wavelengths. Single vibronic level emission spectra obtained by laser excitation of individual levels of the B~ state exhibit two electronic transitions: a very weak, sparse B~–X~ band system in the 450–500 nm region and a stronger, more extensive set of B~2A1–A~2B1 bands in the 580–650 nm region. We have also performed a series of high level ab initio calculations to predict the electronic energies, molecular structures, vibrational frequencies, and rotational and spin-rotation constants in the X~2B2, A~2B1 and B~2A1 electronic states as an aid to the analysis of the experimental data. The theoretical results have been used as input for simulations of the rotationally resolved B~2A1–X~2B2 000 LIF band and Franck-Condon profiles of the LIF and single vibronic level emission spectra. The agreement between the simulations obtained with purely ab initio parameters and the experimental spectra validates the geometries calculated for the ground and excited states and the conclusion that the radical has C2v symmetry in the X~, A~, and B~ states. The spectra provide considerable new information about the vibrational energy levels of the X~ and A~ states, but very little for the B~ state, due to the very restrictive Franck-Condon factors in the LIF spectra
Ab initio studies of O2-(H2O)n and O3-(H2O)n anionic molecular clusters, n≤12
Bork, Nicolai Christian; Kurtén, T.; Enghoff, Martin Andreas Bødker;
2011-01-01
An ab initio study of gaseous clusters of O2− and O2− with water is presented. Based on thorough scans of configurational space, we determine the thermodynamics of cluster growth. The results are in good agreement with benchmark computational methods and existing experimental data. We find...
Ab initio studies of O-2(-) (H2O)(n) and O-3(-) (H2O)(n) anionic molecular clusters, n
Bork, Nicolai Christian; Kurten, T.; Enghoff, Martin Andreas Bødker;
2011-01-01
An ab initio study of gaseous clusters of O-2(-) and O-3(-) with water is presented. Based on thorough scans of configurational space, we determine the thermodynamics of cluster growth. The results are in good agreement with benchmark computational methods and existing experimental data. We find...
The finite-temperature phase diagram of hydrogen in the region of phase IV and its neighborhood was studied using the ab initio molecular dynamics (MD) and the ab initio path-integral molecular dynamics (PIMD). The electronic structures were analyzed using the density-functional theory (DFT), the random-phase approximation, and the diffusion Monte Carlo (DMC) methods. Taking the state-of-the-art DMC results as benchmark, comparisons of the energy differences between structures generated from the MD and PIMD simulations, with molecular and dissociated hydrogens, respectively, in the weak molecular layers of phase IV, indicate that standard functionals in DFT tend to underestimate the dissociation barrier of the weak molecular layers in this mixed phase. Because of this underestimation, inclusion of the quantum nuclear effects (QNEs) in PIMD using electronic structures generated with these functionals leads to artificially dissociated hydrogen layers in phase IV and an error compensation between the neglect of QNEs and the deficiencies of these functionals in standard ab initio MD simulations exists. This analysis partly rationalizes why earlier ab initio MD simulations complement so well the experimental observations. The temperature and pressure dependencies for the stability of phase IV were also studied in the end and compared with earlier results
Chen, Ji; Ren, Xinguo; Li, Xin-Zheng; Alfè, Dario; Wang, Enge
2014-07-14
The finite-temperature phase diagram of hydrogen in the region of phase IV and its neighborhood was studied using the ab initio molecular dynamics (MD) and the ab initio path-integral molecular dynamics (PIMD). The electronic structures were analyzed using the density-functional theory (DFT), the random-phase approximation, and the diffusion Monte Carlo (DMC) methods. Taking the state-of-the-art DMC results as benchmark, comparisons of the energy differences between structures generated from the MD and PIMD simulations, with molecular and dissociated hydrogens, respectively, in the weak molecular layers of phase IV, indicate that standard functionals in DFT tend to underestimate the dissociation barrier of the weak molecular layers in this mixed phase. Because of this underestimation, inclusion of the quantum nuclear effects (QNEs) in PIMD using electronic structures generated with these functionals leads to artificially dissociated hydrogen layers in phase IV and an error compensation between the neglect of QNEs and the deficiencies of these functionals in standard ab initio MD simulations exists. This analysis partly rationalizes why earlier ab initio MD simulations complement so well the experimental observations. The temperature and pressure dependencies for the stability of phase IV were also studied in the end and compared with earlier results. PMID:25028021
Garrone, E.; Ugliengo, P.; Ghiotti, G.; Borello, E.; Saunders, V. R.
1993-08-01
High-quality ab initio computational results for the interaction of N 2O with the silanol molecule H 3SiOH, taken as representative of the isolated hydroxyl at the silica surface, are compared with IR data concerning the real system measured at 195 K. Besides a non-specific interaction, features typical of hydrogen bonding are observed: two types of complexes are formed simultaneously, differing in their 2 v1 mode, probably one nitrogen-bonded, the other oxygen-bonded. Ab initio results are in excellent agreement as far as binding energies are concerned: the two complexes show comparable stabilities, once electron correlation has been taken into account; however, SCF-calculated IR features do not allow a definite vibrational assignment of the two complexes.
Ab initio no core full configuration approach for light nuclei
Kim, Youngman; Shin, Ik Jae; Maris, Pieter; Vary, James P.; Forssén, Christian; Rotureau, Jimmy
2015-10-01
Comprehensive understanding of the structure and reactions of light nuclei poses theoretical and computational challenges. Still, a number of ab initio approaches have been developed to calculate the properties of atomic nuclei using fundamental interactions among nucleons. Among them, we work with the ab initio no core full configuration (NCFC) method and ab initio no core Gamow Shell Model (GSM). We first review these approaches and present some recent results.
Ab Initio Calculations of Oxosulfatovanadates
Frøberg, Torben; Johansen, Helge
1996-01-01
Restricted Hartree-Fock and multi-configurational self-consistent-field calculations together with secondorder perturbation theory have been used to study the geometry, the electron density, and the electronicspectrum of (VO2SO4)-. A bidentate sulphate attachment to vanadium was found to be stable...... with anO-V-O angle of 72.5 degrees . The calculated spectrum shows bands in reasonable agreement with anexperimental spectrum which has been attributed to (VO2SO4)-. The geometry and the electron density fortwo binuclear vanadium complexes proposed as intermediates in the vanadium catalyzed SO2...
Germacrene D Cyclization: An Ab Initio Investigation
William N. Setzer
2008-01-01
Full Text Available Essential oils that contain large concentrations of germacrene D are typically accompanied by cadinane sesquiterpenoids. The acid-catalyzed cyclization of germacrene D to give cadinane and selinane sesquiterpenes has been computationally investigated using both density functional (B3LYP/6-31G* and post Hartree-Fock (MP2/6-31G** ab initio methods. The calculated energies are in general agreement with experimentally observed product distributions, both from acid-catalyzed cyclizations as well as distribution of the compounds in essential oils.
Ab initio calculations of material strength
Šob, Mojmír; Friák, Martin; Vitek, V.
Tokyo : The Japan Society of Mechanical Engineers, 2003, s. 467-475. [International Symposium on Micro-Mechanical Engineering - Heat Transfer, Fluid Dynamics, Reliability and Mechanotronics.. Tsuchiura and Tsukuba (JP), 01.12.2003-03.12.2003] R&D Projects: GA AV ČR IAA1041302; GA ČR GA202/03/1351; GA MŠk OC 523.90 Institutional research plan: CEZ:AV0Z2041904 Keywords : ab initio calculations * electronic structure * theoretical tensile strength Subject RIV: BM - Solid Matter Physics ; Magnetism
Ab Initio Molecular Dynamics: A Virtual Laboratory
Hobbi Mobarhan, Milad
2014-01-01
In this thesis, we perform ab initio molecular dynamics (MD) simulations at the Hartree-Fock level, where the forces are computed on-the-fly using the Born-Oppenheimer approximation. The theory behind the Hartree-Fock method is discussed in detail and an implementation of this method based on Gaussian basis functions is explained. We also demonstrate how to calculate the analytic energy derivatives needed for obtaining the forces acting on the nuclei. Hartree-Fock calculations on the ground s...
Ab initio non-relativistic spin dynamics
Ding, Feizhi; Goings, Joshua J.; Li, Xiaosong, E-mail: xsli@uw.edu [Department of Chemistry, University of Washington, Seattle, Washington 98195 (United States); Frisch, Michael J. [Gaussian, Inc., 340 Quinnipiac St, Bldg 40, Wallingford, Connecticut 06492 (United States)
2014-12-07
Many magnetic materials do not conform to the (anti-)ferromagnetic paradigm where all electronic spins are aligned to a global magnetization axis. Unfortunately, most electronic structure methods cannot describe such materials with noncollinear electron spin on account of formally requiring spin alignment. To overcome this limitation, it is necessary to generalize electronic structure methods and allow each electron spin to rotate freely. Here, we report the development of an ab initio time-dependent non-relativistic two-component spinor (TDN2C), which is a generalization of the time-dependent Hartree-Fock equations. Propagating the TDN2C equations in the time domain allows for the first-principles description of spin dynamics. A numerical tool based on the Hirshfeld partitioning scheme is developed to analyze the time-dependent spin magnetization. In this work, we also introduce the coupling between electron spin and a homogenous magnetic field into the TDN2C framework to simulate the response of the electronic spin degrees of freedom to an external magnetic field. This is illustrated for several model systems, including the spin-frustrated Li{sub 3} molecule. Exact agreement is found between numerical and analytic results for Larmor precession of hydrogen and lithium atoms. The TDN2C method paves the way for the ab initio description of molecular spin transport and spintronics in the time domain.
QIONG WU; DONG XIANG; GUOLIN XIONG; WEIHUA ZHU; HEMING XIAO
2016-05-01
Ab initio molecular dynamics simulations were performed to study the initiation of decompositionand formation of first products of two molecular crystals pentaerythritol tetranitrate (PETN) and 5-nitro-2,4-dihydro-1,2,4-triazole-3-one (NTO) under thermal decomposition temperature (475 K for PETN and 531 Kfor NTO) coupled with different pressures (1-5 GPa). The pressure effects on the initial decomposition stepsand initially generated products on PETN and NTO were very different. PETN was triggered by C-H... O intermolecular hydrogen transfer. The initial decomposition mechanism was independent of the pressure. ForNTO, two different initial decomposition mechanisms were found. At 1, 2, and 3 GPa, it was triggered by NH....O intermolecular hydrogen transfer, while at 4 and 5 GPa, it was triggered by N-H.....N intermolecularhydrogen transfer. This indicates that the initial decomposition mechanism was dependent on the pressure.Our study may provide new insights into initial mechanisms and decomposition reactions of molecular crystalexplosives under thermal decomposition temperature coupled with different pressures with details at atomiclevel.
Ab initio studies on [bmim][PF6]–CO2 mixture and CO2 clusters
B L Bhargava; M Saharay; S Balasubramanian
2008-06-01
Ab initio molecular dynamics studies have been carried out on the room temperature ionic liquid, 1,n-butyl,3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and supercritical carbon dioxide mixture at room temperature and experimental density. Partial radial distribution functions (RDF) for different sites have been computed to see the organization of CO2 molecules around the ionic liquid. Several partial RDFs around the carbon atom of CO2 molecule are compared to find out that the CO2 has specific interaction with a carbon atom present in the imidazolium ring. The CO2 is also found to be very well organized around the terminal carbon atom of the butyl chain. The partial RDFs for the oxygen atoms around oxygen and carbon atoms of the CO2 suggests that there is very good organization of CO2 molecules around themselves even in the [bmim][PF6] – CO2 mixture. The instantaneous quadrupole moment tensor has been calculated for the anion and the cation. The ensemble average of diagonal components of quadrupole moment tensor of the cation have finite values, whereas the off-diagonal components of the cation and both the diagonal and off-diagonal components of the anion have the value of zero with a large standard deviation. The CPMD studies performed on CO2 clusters reveals the greater tendency of the clusters with more CO2 units, to deviate from the linear geometry.
Ab initio study of Kr in hcp Ti: Diffusion, formation and stability of small Kr-vacancy clusters
Ab initio electronic structure calculations have been performed to study the formation and migration of Kr impurities, and the stability of small Kr-vacancy clusters for clusters with up to four vacancies and four Kr atoms, in hcp Ti. Both the substitutional and the interstitial configurations of Kr are found to be stable. The octahedral configuration is however found to be more stable than the tetrahedral. Interstitial Kr atoms are shown to have attractive interactions and a low migration barrier, suggesting that, at low temperature, Kr bubble formation is possible, even in the absence of vacancies. We also find vacancy clusters to be stable. The binding energies of an interstitial Kr atom and a vacancy to a Kr-vacancy cluster are obtained from the calculated formation energies of the clusters. The stability of small-vacancy clusters is found to be dependent on Kr-vacancy ratio. The trends of the calculated binding energies are discussed in terms of providing further insights on the behaviour of Kr in implanted Ti.
The electronic transitions from the ground state to low-lying excited states of CF3OH have been investigated using high level ab initio quantum mechanical techniques. Also the possible photo dissociation procedures of CF3OH have been considered. The highest level employed in this study is TZP CCSD(T) level of theory. The possible four low-lying excited states can result by the excitation of the lone pair electron (n) in oxygen to σ molecular orbital in C-O or O-H bond. The vertical transition (n→σ) energy is predicted to be 220.5 kcal/mol (130 nm) at TZ2P CISD level to theory. The bond dissociation energies of CF3OH to CF3O + H and CF3+OH have been predicted to be 119.5 kcal/mol 114.1 kcal/mol, respectively, at TZP CCSD level of theory. In addition, the transition state for the unimolecular decomposition of CF3OH into CF2O + HF has been examined. The activation energy and energy separation for this decomposition have been computed to be 43.6 kcal/mol and 5.0 kcal/mol including zero-point vibrational energy corrections at TZP CCSD(T) level of theory
Ab initio and atomistic study of generalized stacking fault energies in Mg and Mg–Y alloys
Magnesium–yttrium alloys show significantly improved room temperature ductility when compared with pure Mg. We study this interesting phenomenon theoretically at the atomic scale employing quantum-mechanical (so-called ab initio) and atomistic modeling methods. Specifically, we have calculated generalized stacking fault energies for five slip systems in both elemental magnesium (Mg) and Mg–Y alloys using (i) density functional theory and (ii) a set of embedded-atom-method (EAM) potentials. These calculations predict that the addition of yttrium results in a reduction in the unstable stacking fault energy of basal slip systems. Specifically in the case of an I2 stacking fault, the predicted reduction of the stacking fault energy due to Y atoms was verified by experimental measurements. We find a similar reduction for the stable stacking fault energy of the {11 2-bar 2} non-basal slip system. On the other hand, other energies along this particular γ-surface profile increase with the addition of Y. In parallel to our quantum-mechanical calculations, we have also developed a new EAM Mg–Y potential and thoroughly tested its performance. The comparison of quantum-mechanical and atomistic results indicates that the new potential is suitable for future large-scale atomistic simulations. (paper)
The mechanical properties of LixCoO2 under various Li concentrations and associated anisotropy have been systematically studied using the first principles method. During the lithium intercalation process, the Young's modulus, bulk modulus, shear modulus, and ultimate strength increase with increasing lithium concentration. Strong anisotropy of mechanical properties between a-axis and c-axis in LixCoO2 is identified at low lithium concentrations, and the anisotropy decreases with increasing lithium concentration. The observed lithium concentration dependence and anisotropy are explained by analyzing the charge transfer using Bader charge analysis, bond order analysis, and bond strength by investigating partial density of states and charge density difference. With the decrease of Li concentration, the charge depletion in the bonding regions increases, indicating a weaker Co-O bond strength. Additionally, the Young's modulus, bulk modulus, shear modulus, and toughness are obtained by simulating ab initio tensile tests. From the simulated stress-strain curves, LixCoO2 shows the highest toughness, which is in contraction with Pugh criterion prediction based on elastic properties only
Bucci, F.; Sanna, A.; Continenza, A.; Katrych, S.; Karpinski, J.; Gross, E. K. U.; Profeta, G.
2016-01-01
As a follow-up to the discovery of a new family of Fe-based superconductors, namely, the RE4Fe2As2Te1 -xO4 (42214) (RE = Pr, Sm, and Gd), we present a detailed ab initio study of these compounds highlighting the role of rare-earth (RE) atoms, external pressure, and Te content on their physical properties. Modifications of the structural, magnetic, and electronic properties of the pure (e.g., x =0.0 ) 42214 compounds and their possible correlations with the observed superconducting properties are calculated and discussed. The careful analysis of the results obtained shows that (i) changing the RE atoms allows one to tune the internal pressure acting on the As height with respect to the Fe planes; (ii) similarly to other Fe pnictides, the 42214 pure compounds show an antiferromagnetic-stripe magnetic ground state phase joined by an orthorhombic distortion (not experimentally found yet); (iii) smaller RE atoms increase the magnetic instability of the compounds possibly favoring the onset of the superconducting state; (iv) external pressure induces the vanishing of the magnetic order with a transition to the tetragonal phase and can be a possible experimental route towards higher superconducting critical temperature (Tc) ; and (v) Te vacancies act on the structural parameters, changing the As height and affecting the stability of the magnetic phase.
Ab initio study of the one-dimensional H-bonded ferroelectric CsH2PO4
Lasave, J.; Abufager, P.; Koval, S.
2016-04-01
We studied the microscopic mechanism of the paraelectric-ferroelectric (PE-FE) phase transition of CsH2PO4 (CDP) by means of first-principles electronic structure calculations. The calculated structural parameters in the PE and FE phases as well as the total spontaneous polarization Ps obtained with the Berry phase formalism for CDP are in good agreement with experiments. The main contribution to Ps originates from a large y x component of the calculated Born effective-charge tensor for the disordered protons. Moreover, this component is ≈2.7 times larger than the z x component of the proton effective-charge tensor relevant to the polarization in the H-bonded FE KH2PO4 (KDP). This is the main feature that compensates the different number of protons per formula unit involved in the phase transitions for CDP and KDP to give close values for their measured Ps. Correlations among protons and heavy atoms along chains in the b direction lead to larger instabilities for the global and local FE distortions in CDP and its deuterated counterpart DCDP. We conclude that the tunneling particle is a dressed proton (deuteron) in agreement with experiments and with recent ab initio results for KDP.
Ab Initio Study of Water Adsorption and Reactivity on the (211) Surface of Anatase TiO2
Xu, Jing; Xu, Li-Fang; Li, Zhen-Zhen; Wang, Jian-Tao; Lin, Zhe-Shuai; Liu, Kai; Cao, Yong-Ge; Selloni, Annabella
2016-06-01
The reactivity of the anatase TiO2 (211) surface is systematically studied by ab initio calculations of the surface energy and water-adsorption energy. We find that anatase (211) has a high surface energy of 0.97 J /m2 , close to that of the (001) surface, and the unsaturated fourfold-coordinated Ti4 atom is more reactive than the unsaturated fivefold-coordinated Ti5 atom. Accordingly, for water adsorption on the (211) surface, a dissociative form is favored on Ti4 sites, with a large adsorption energy Δ HH ,OH˜1.28 eV , while a nondissociative molecular form is favored on Ti5 sites, with a smaller adsorption energy Δ HH2O˜0.78 eV . Such distinct surface properties lead to a mixed dissociative and molecular adsorption configuration when the coverage is increased from 1 /3 to 1 monolayer. These results suggest that, similar to the (001) surface, the anatase (211) surface exhibits high reactivity and may be useful in catalytic and photocatalytic applications as well.
A comparative ab initio study of superconductivity in the body centered tetragonal YC2 and LaC2
Ab initio studies of the electronic band structure and phonon dispersion relations, using the planewave pseudopotential method and the density functional theory, have been made for the superconducting materials YC2 and LaC2. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges between these materials are investigated and discussed. By integrating the Eliashberg spectral function α2F(ω), the average electron-phonon coupling parameter is found to be λ = 0.55 for YC2 and 0.54 for LaC2, indicating these to be weak-coupling BCS superconductors. It is established that about 60% of λ is contributed by acoustic phonons in both materials. Using a reasonable value of μ* = 0.13 for the effective Coulomb repulsion parameter, the superconducting critical temperature Tc is found to be 3.81 K for YC2 and 2.44 K for LaC2, in good agreement with values reported from experimental measurements
Using density functional theory and Ab Initio Molecular Dynamics with Electronic Friction (AIMDEF), we study the adsorption and dissipative vibrational dynamics of hydrogen atoms chemisorbed on free-standing lead films of increasing thickness. Lead films are known for their oscillatory behaviour of certain properties with increasing thickness, e.g., energy and electron spillout change in discontinuous manner, due to quantum size effects [G. Materzanini, P. Saalfrank, and P. J. D. Lindan, Phys. Rev. B 63, 235405 (2001)]. Here, we demonstrate that oscillatory features arise also for hydrogen when chemisorbed on lead films. Besides stationary properties of the adsorbate, we concentrate on finite vibrational lifetimes of H-surface vibrations. As shown by AIMDEF, the damping via vibration-electron hole pair coupling dominates clearly over the vibration-phonon channel, in particular for high-frequency modes. Vibrational relaxation times are a characteristic function of layer thickness due to the oscillating behaviour of the embedding surface electronic density. Implications derived from AIMDEF for frictional many-atom dynamics, and physisorbed species will also be given
Saalfrank, Peter [Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, D-14476 Potsdam (Germany); Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián (Spain); Juaristi, J. I. [Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián (Spain); Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), Paseo Manuel de Lardizabal 5, 20018 Donostia-San Sebastián (Spain); Departamento de Física de Materiales, Facultad de Químicas UPV/EHU, Apartado 1072, 20018 Donostia-San Sebastián (Spain); Alducin, M.; Muiño, R. Díez [Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián (Spain); Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), Paseo Manuel de Lardizabal 5, 20018 Donostia-San Sebastián (Spain); Blanco-Rey, M. [Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián (Spain); Departamento de Física de Materiales, Facultad de Químicas UPV/EHU, Apartado 1072, 20018 Donostia-San Sebastián (Spain)
2014-12-21
Using density functional theory and Ab Initio Molecular Dynamics with Electronic Friction (AIMDEF), we study the adsorption and dissipative vibrational dynamics of hydrogen atoms chemisorbed on free-standing lead films of increasing thickness. Lead films are known for their oscillatory behaviour of certain properties with increasing thickness, e.g., energy and electron spillout change in discontinuous manner, due to quantum size effects [G. Materzanini, P. Saalfrank, and P. J. D. Lindan, Phys. Rev. B 63, 235405 (2001)]. Here, we demonstrate that oscillatory features arise also for hydrogen when chemisorbed on lead films. Besides stationary properties of the adsorbate, we concentrate on finite vibrational lifetimes of H-surface vibrations. As shown by AIMDEF, the damping via vibration-electron hole pair coupling dominates clearly over the vibration-phonon channel, in particular for high-frequency modes. Vibrational relaxation times are a characteristic function of layer thickness due to the oscillating behaviour of the embedding surface electronic density. Implications derived from AIMDEF for frictional many-atom dynamics, and physisorbed species will also be given.
Kopplung von Dichtefunktional- und ab-initio-Methoden
Goll, Erich
2008-01-01
Im Rahmen der Doktorarbeit wurde untersucht, inwieweit die Kopplung von Dichtefunktionalmethoden und ab-initio-Korrelationsmethoden der Quantenchemie eine Verbesserung bezüglich beider Grenzmethoden erbringt. Die Kopplung erfolgt durch eine Aufspaltung des interelektronischen Hamiltonoperators (abstoßende Coulombwechselwirkung). Die kurzreichweitige Wechselwirkung wird mit Dichtefunktionaltheorie behandelt, die langreichweitige mit Hilfe von ab-initio-Methoden. Diese Aufteilung soll dazu dien...
Wang, Zi; Bevan, Kirk H.
2016-01-01
In the present work, we study the effects of the electronic relaxation of semicore levels on polaron activation energies and dynamics. Within the framework of adiabatic ab initio theory, we utilize both static transition state theory and molecular dynamics methods for an in-depth study of polaronic hopping in delithiated LiFePO4 (FePO4). Our results show that electronic relaxation of semicore states is significant in FePO4, resulting in a lower activation barrier and kinetics that is one to two orders faster compared to the result of calculations that do not incorporate semicore states. In general, the results suggest that the relaxation of states far below the Fermi energy could dramatically impact the ab initio polaronic barrier estimates for many transition metal oxides and phosphates.
The temperature dependence of the effective Waldmann--Snider cross sections determining the Senftleben--Beenakker effects of viscosity and heat conductivity has been studied for pH2 gas between 10 and 200 K. From ab initio nonspherical potentials of H2--H2, scattering matrices have been determined in close-coupling calculations. From these, the elements of the scattering amplitude matrix have been obtained and used as input quantities for the evaluation of the various Waldmann--Snider collision integrals. The results of these first ab initio numerical calculations of anisotropic transport coefficients show excellent agreement of calculated and measured effective cross sections, especially for the most recent improved version of the interaction potential. In addition, it has been shown that the polarization production cross sections are quite sensitive to the potential anisotropy
Zendlová, Lucie; Hobza, Pavel; Kabeláč, Martin
2007-01-01
Roč. 111, č. 10 (2007), s. 2591-2609. ISSN 1520-6106 R&D Projects: GA MŠk LC512; GA ČR GA203/05/0009; GA ČR(CZ) GD203/05/H001; GA AV ČR KJB400550518 Institutional research plan: CEZ:AV0Z40550506 Keywords : ab initio calculation * organic solvent * nucleobases * H-bonding * stacking Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 4.086, year: 2007
G.M. Bhuiyan
2012-10-01
Full Text Available Several static and dynamic properties of liquid Cu, Ag and Au at thermodynamic states near their respective melting points, have been evaluated by means of the orbital free ab-initio molecular dynamics simulation method. The calculated static structure shows good agreement with the available X-ray and neutron diffraction data. As for the dynamic properties, the calculated dynamic structure factors point to the existence of collective density excitations along with a positive dispersion for l-Cu and l-Ag. Several transport coefficients have been obtained which show a reasonable agreement with the available experimental data.
Iolanta I. Balan; Natalia N. Gorinchoy
2011-01-01
The four-stage mechanism of reaction of the rhodium trihydride complex [(triphos)RhH3] (triphos=1,1,1-tris(diphenylphosphanylmethyl)ethane) with the white phosphorus molecule resulting in the phosphane and the cyclo-P3 complex [(triphos)M(η3-P3] is analyzed on the basis of ab initio calculations of reactants, products, and intermediate complexes of reaction. It is shown that generation of the transient complex [(triphos)RhH(η1:η1-P4)] followed by intramolecular hydrogen atom migration from t...
Vonci, Michele; Giansiracusa, Marcus J; Gable, Robert W; Van den Heuvel, Willem; Latham, Kay; Moubaraki, Boujemaa; Murray, Keith S; Yu, Dehong; Mole, Richard A; Soncini, Alessandro; Boskovic, Colette
2016-02-01
Ab initio calculations carried out on the Tb analogue of the single-molecule magnet family Na9[Ln(W5O18)2] (Ln = Nd, Gd, Ho and Er) have allowed interpretation of the inelastic neutron scattering spectra. The combined experimental and theoretical approach sheds new light on the sensitivity of the electronic structure of the Tb(III) ground and excited states to small structural distortions from axial symmetry, thus revealing the subtle relationship between molecular geometry and magnetic properties of the two isostructural species that comprise the sample. PMID:26690503
Siraleartmukul, Krisana; Siriwong, Khatcharin; Remsungnen, Tawun; Muangsin, Nongnuj; Udomkichdecha, Werasak; Hannongbua, Supot
2004-09-01
The solvation structure of glucosamine in aqueous solution was investigated using Monte Carlo simulation at 298 K. The MCY rigid water model and ab initio glucosamine-water fitted potential were applied. The first hydration shell appears at 4.6 Å from the center of glucosamine with a coordination number of seven water molecules where one water lies in the ligand's plane while two and four of them are about 2-4 Å above and below the plane, respectively. Furthermore, the mobility distribution and orientation of the water molecules around the ligand have been intensively investigated and reported.
Magnetism in zigzag and armchair CuO nanotubes: Ab-initio study
Paudel, Samir; Dandeliya, Sushmita; Chaurasiya, Rajneesh; Srivastava, Anurag; Kaphle, Gopi Chandra
2016-05-01
The structural stability, electronic band structure and magnetic properties of zigzag (4≤n≤12) and armchair (3≤n≤8) copper oxide nanotubes have been analyzed by employing a standard Density Functional Theory based ab-intio approach using spin polarized generalized gradient approximation with revised Perdew Burke Ernzerhoff type parameterization. The binding energy of both zigzag as well as armchair CuO nanotubes increases with increasing diameter. Whereas, calculated total magnetic moment decreases with increasing diameter. Among the considered chiralities, (5, 5) armchair (diameter 9.08 Å) and (9, 0) zigzag (diameter 9.47 Å) CuO nanotubes show highest degree of spin polarization, however total magnetic moment is found to be highest for (4, 0) zigzag and (3, 3) armchair CuO nanotubes. The computed electronic properties of considered CuO Nanotube, confirms the metallic nature of these nanotubes.
Ab initio simulation of helium inside carbon nanotubes
In present work we consider the complex behaviour of quantum liquids like liquid He-4 inside carbon nanotubes. Interactions between helium atoms and carbon atoms of the short-length atomistic model and model with periodical boundary conditions of carbon nanotube were studied via ab initio quantum simulations. Effects of geometrical confinement of the tube on the He behaviour inside CNT (13,0) have been explored. Nanotubes with typical average diameter of 10 angstroms are under consideration.
Ab Initio Modeling of Ecosystems with Artificial Life
Adami, C.
2002-01-01
Artificial Life provides the opportunity to study the emergence and evolution of simple ecosystems in real time. We give an overview of the advantages and limitations of such an approach, as well as its relation to individual-based modeling techniques. The Digital Life system Avida is introduced and prospects for experiments with ab initio evolution (evolution "from scratch"), maintenance, as well as stability of ecosystems are discussed.
Augmented wave ab initio EFG calculations: some methodological warnings
Errico, Leonardo A. [Departamento de Fisica-IFLP (CONICET), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67 (1900) La Plata (Argentina); Renteria, Mario [Departamento de Fisica-IFLP (CONICET), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67 (1900) La Plata (Argentina); Petrilli, Helena M. [Instituto de Fisica-DFMT, Universidade de Sao Paulo, C.P. 66318, 05315-970 Sao Paulo, SP (Brazil)]. E-mail: hmpetril@macbeth.if.usp.br
2007-02-01
We discuss some accuracy aspects inherent to ab initio electronic structure calculations in the understanding of nuclear quadrupole interactions. We use the projector augmented wave method to study the electric-field gradient (EFG) at both Sn and O sites in the prototype cases SnO and SnO{sub 2}. The term ab initio is used in the standard context of the also called first principles methods in the framework of the Density Functional Theory. As the main contributions of EFG calculations to problems in condensed matter physics are related to structural characterizations on the atomic scale, we discuss the 'state of the art' on theoretical EFG calculations and make a brief critical review on the subject, calling attention to some fundamental theoretical aspects.
Augmented wave ab initio EFG calculations: some methodological warnings
We discuss some accuracy aspects inherent to ab initio electronic structure calculations in the understanding of nuclear quadrupole interactions. We use the projector augmented wave method to study the electric-field gradient (EFG) at both Sn and O sites in the prototype cases SnO and SnO2. The term ab initio is used in the standard context of the also called first principles methods in the framework of the Density Functional Theory. As the main contributions of EFG calculations to problems in condensed matter physics are related to structural characterizations on the atomic scale, we discuss the 'state of the art' on theoretical EFG calculations and make a brief critical review on the subject, calling attention to some fundamental theoretical aspects
Ab-initio calculations on melting of thorium
Mukherjee, D.; Sahoo, B. D.; Joshi, K. D.; Kaushik, T. C.; Gupta, Satish C.
2016-05-01
Ab-initio molecular dynamics study has been performed on face centered cubic structured thorium to determine its melting temperature at room pressure. The ion-electron interaction potential energy calculated as a function of temperature for three volumes (a0)3 and (1.02a0)3 and (1.04a0)3 increases gradually with temperature and undergoes a sharp jump at ~2200 K, ~2100 K and ~1800 K, respectively. Here, a0 = 5.043 Å is the equilibrium lattice parameter at 0 K obtained from ab-initio calculations. These jumps in interaction energy are treated as due to the onset of melting and corresponding temperatures as melting point. The melting point of 2100 K is close to the experimental value of 2023K. Further, the same has been verified by plotting the atomic arrangement evolved at various temperatures and corresponding pair correlation functions.
Guiding ab initio calculations by alchemical derivatives
to Baben, M.; Achenbach, J. O.; von Lilienfeld, O. A.
2016-03-01
We assess the concept of alchemical transformations for predicting how a further and not-tested change in composition would change materials properties. This might help to guide ab initio calculations through multidimensional property-composition spaces. Equilibrium volumes, bulk moduli, and relative lattice stability of fcc and bcc 4d transition metals Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag are calculated using density functional theory. Alchemical derivatives predict qualitative trends in lattice stability while equilibrium volumes and bulk moduli are predicted with less than 9% and 28% deviation, respectively. Predicted changes in equilibrium volume and bulk moduli for binary and ternary mixtures of Rh-Pd-Ag are in qualitative agreement even for predicted bulk modulus changes as large as +100% or -50%. Based on these results, it is suggested that alchemical transformations could be meaningful for enhanced sampling in the context of virtual high-throughput materials screening projects.
Ab Initio Path to Heavy Nuclei
Binder, Sven; Calci, Angelo; Roth, Robert
2014-01-01
We present the first ab initio calculations of nuclear ground states up into the domain of heavy nuclei, spanning the range from 16-O to 132-Sn based on two- plus three-nucleon interactions derived within chiral effective field theory. We employ the similarity renormalization group for preparing the Hamiltonian and use coupled-cluster theory to solve the many-body problem for nuclei with closed sub-shells. Through an analysis of theoretical uncertainties resulting from various truncations in this framework, we identify and eliminate the technical hurdles that previously inhibited the step beyond medium-mass nuclei, allowing for reliable validations of nuclear Hamiltonians in the heavy regime. Following this path we show that chiral Hamiltonians qualitatively reproduce the systematics of nuclear ground-state energies up to the neutron-rich Sn isotopes.
Operator evolution for ab initio nuclear theory
Schuster, Micah D; Johnson, Calvin W; Jurgenson, Eric D; Navratil, Petr
2014-01-01
The past two decades have seen a revolution in ab initio calculations of nuclear properties. One key element has been the development of a rigorous effective interaction theory, applying unitary transformations to soften the nuclear Hamiltonian and hence accelerate the convergence as a function of the model space size. For consistency, however, one ought to apply the same transformation to other operators when calculating transitions and mean values from the eigenstates of the renormalized Hamiltonian. Working in a translationally-invariant harmonic oscillator basis for the two- and three-nucleon systems, we evolve the Hamiltonian, square-radius and total dipole strength operators by the similarity renormalization group (SRG). The inclusion of up to three-body matrix elements in the 4He nucleus all but completely restores the invariance of the expectation values under the transformation. We also consider a Gaussian operator with adjustable range and find at short ranges an increased contribution from such ind...
Discovering chemistry with an ab initio nanoreactor
Martinez, Todd
Traditional approaches for modeling chemical reaction networks such as those involved in combustion have focused on identifying individual reactions and using theoretical approaches to explore the underlying mechanisms. Recent advances involving graphical processing units (GPUs), commodity products developed for the videogaming industry, have made it possible to consider a distinct approach wherein one attempts to discover chemical reactions and mechanisms. We provide a brief summary of these developments and then discuss the concept behind the ``ab initio nanoreactor'' which explores the space of possible chemical reactions and molecular species for a given stoichiometry. The nanoreactor concept is exemplified with an example to the Urey-Miller reaction network which has been previously advanced as a potential model for prebiotic chemistry. We briefly discuss some of the future directions envisioned for the development of this nanoreactor concept.
Ab initio molar volumes and Gaussian radii.
Parsons, Drew F; Ninham, Barry W
2009-02-12
Ab initio molar volumes are calculated and used to derive radii for ions and neutral molecules using a spatially diffuse model of the electron distribution with Gaussian spread. The Gaussian radii obtained can be used for computation of nonelectrostatic ion-ion dispersion forces that underlie Hofmeister specific ion effects. Equivalent hard-sphere radii are also derived, and these are in reasonable agreement with crystalline ionic radii. The Born electrostatic self-energy is derived for a Gaussian model of the electronic charge distribution. It is shown that the ionic volumes used in electrostatic calculations of strongly hydrated cosmotropic ions ought best to include the first hydration shell. Ionic volumes for weakly hydrated chaotropic metal cations should exclude electron overlap (in electrostatic calculations). Spherical radii are calculated as well as nonisotropic ellipsoidal radii for nonspherical ions, via their nonisotropic static polarizability tensors. PMID:19140766
Ab initio alpha-alpha scattering.
Elhatisari, Serdar; Lee, Dean; Rupak, Gautam; Epelbaum, Evgeny; Krebs, Hermann; Lähde, Timo A; Luu, Thomas; Meißner, Ulf-G
2015-12-01
Processes such as the scattering of alpha particles ((4)He), the triple-alpha reaction, and alpha capture play a major role in stellar nucleosynthesis. In particular, alpha capture on carbon determines the ratio of carbon to oxygen during helium burning, and affects subsequent carbon, neon, oxygen, and silicon burning stages. It also substantially affects models of thermonuclear type Ia supernovae, owing to carbon detonation in accreting carbon-oxygen white-dwarf stars. In these reactions, the accurate calculation of the elastic scattering of alpha particles and alpha-like nuclei--nuclei with even and equal numbers of protons and neutrons--is important for understanding background and resonant scattering contributions. First-principles calculations of processes involving alpha particles and alpha-like nuclei have so far been impractical, owing to the exponential growth of the number of computational operations with the number of particles. Here we describe an ab initio calculation of alpha-alpha scattering that uses lattice Monte Carlo simulations. We use lattice effective field theory to describe the low-energy interactions of protons and neutrons, and apply a technique called the 'adiabatic projection method' to reduce the eight-body system to a two-cluster system. We take advantage of the computational efficiency and the more favourable scaling with system size of auxiliary-field Monte Carlo simulations to compute an ab initio effective Hamiltonian for the two clusters. We find promising agreement between lattice results and experimental phase shifts for s-wave and d-wave scattering. The approximately quadratic scaling of computational operations with particle number suggests that it should be possible to compute alpha scattering and capture on carbon and oxygen in the near future. The methods described here can be applied to ultracold atomic few-body systems as well as to hadronic systems using lattice quantum chromodynamics to describe the interactions of
Ab initio alpha-alpha scattering
Elhatisari, Serdar; Lee, Dean; Rupak, Gautam; Epelbaum, Evgeny; Krebs, Hermann; Lähde, Timo A.; Luu, Thomas; Meißner, Ulf-G.
2015-12-01
Processes such as the scattering of alpha particles (4He), the triple-alpha reaction, and alpha capture play a major role in stellar nucleosynthesis. In particular, alpha capture on carbon determines the ratio of carbon to oxygen during helium burning, and affects subsequent carbon, neon, oxygen, and silicon burning stages. It also substantially affects models of thermonuclear type Ia supernovae, owing to carbon detonation in accreting carbon-oxygen white-dwarf stars. In these reactions, the accurate calculation of the elastic scattering of alpha particles and alpha-like nuclei—nuclei with even and equal numbers of protons and neutrons—is important for understanding background and resonant scattering contributions. First-principles calculations of processes involving alpha particles and alpha-like nuclei have so far been impractical, owing to the exponential growth of the number of computational operations with the number of particles. Here we describe an ab initio calculation of alpha-alpha scattering that uses lattice Monte Carlo simulations. We use lattice effective field theory to describe the low-energy interactions of protons and neutrons, and apply a technique called the ‘adiabatic projection method’ to reduce the eight-body system to a two-cluster system. We take advantage of the computational efficiency and the more favourable scaling with system size of auxiliary-field Monte Carlo simulations to compute an ab initio effective Hamiltonian for the two clusters. We find promising agreement between lattice results and experimental phase shifts for s-wave and d-wave scattering. The approximately quadratic scaling of computational operations with particle number suggests that it should be possible to compute alpha scattering and capture on carbon and oxygen in the near future. The methods described here can be applied to ultracold atomic few-body systems as well as to hadronic systems using lattice quantum chromodynamics to describe the interactions of
The solid solution based on Nb5Si3 (Cr5B3 structure type, D8l, tI32, I4/mcm, No140, a=6.5767 Å, c=11.8967 Å) in the Nb–Si–B system was studied from the structural and thermodynamic point of view both experimentally and by ab initio calculations. Rietveld refinement of powder X-ray synchrotron data allowed to determine the boron to silicon substitution mechanism and the structural parameters. Ab initio calculations of different ordered compounds and selected disordered alloys allowed to obtain in addition to the enthalpy of formation of the solution, substitution mechanism and structural parameters which are in excellent agreement with the experimental data. The stability of the phase is discussed. - Graphial abstract: Valence-charge electron localization function in the z=0 plane of the D8l structure for the ordered compound Nb5SiB2. Highlights: ► Coupling between ab initio data and experimental results from synchrotron powder diffraction. ► Excellent agreement between the two techniques for the site occupancies and internal coordinates. ► Explanation of the phase stability up to Nb5SiB2.
Zemen, J.; Mašek, J.; Kučera, J.; Mol, J. A.; Motloch, P.; Jungwirth, T.
2014-04-01
An empirical multiorbital (spd) tight binding (TB) model including magnetism and spin-orbit coupling is applied to calculations of magnetic anisotropy energy (MAE) in CoPt L10 structure. A realistic Slater-Koster parametrisation for single-element transition metals is adapted for the ordered binary alloy. Spin magnetic moment and density of states are calculated using a full-potential linearised augmented plane-wave (LAPW) ab initio method and our TB code with different variants of the interatomic parameters. Detailed mutual comparison of this data allows for determination of a subset of the compound TB parameters tuning of which improves the agreement of the TB and LAPW results. MAE calculated as a function of band filling using the refined parameters is in broad agreement with ab initio data for all valence states and in quantitative agreement with ab initio and experimental data for the natural band filling. Our work provides a practical basis for further studies of relativistic magnetotransport anisotropies by means of local Green's function formalism which is directly compatible with our TB approach.
A theoretical-spectroscopy, ab initio-based study of the electronic ground state of 121SbH3
For the stibine isotopologue 121SbH3, we report improved theoretical calculations of the vibrational energies below 8000 cm-1 and simulations of the rovibrational spectrum in the 0-8000 cm-1 region. The calculations are based on a refined ab initio potential energy surface and on a new dipole moment surface obtained at the coupled cluster CCSD(T) level. The theoretical results are compared with the available experimental data in order to validate the ab initio surfaces and the TROVE computational method [Yurchenko SN, Thiel W, Jensen P. J Mol Spectrosc 2007;245:126-40] for calculating rovibrational energies and simulating rovibrational spectra of arbitrary molecules in isolated electronic states. A number of predicted vibrational energies of 121SbH3 are provided in order to stimulate new experimental investigations of stibine. The local-mode character of the vibrations in stibine is demonstrated through an analysis of the results in terms of local-mode theory.
Rodríguez-Santiago, Luis; Alí-Torres, Jorge; Vidossich, Pietro; Sodupe, Mariona
2015-05-28
Several lines of evidence supporting the role of metal ions in amyloid aggregation, one of the hallmarks of Alzheimer's disease (AD), have turned metal ion chelation into a promising therapeutic treatment. The design of efficient chelating ligands requires proper knowledge of the electronic and molecular structure of the complexes formed, including their hydration properties. Among various potential chelators, clioquinol (5-chloro-7-iodo-8-hydroxyquinoline, CQH) has been evaluated with relative success in in vitro experiments and even in phase 2 clinical trials. Clioquinol interacts with Zn(ii) to lead to a binary metal/ligand 1 : 2 stoichiometric complex in which the phenolic group of CQH is deprotonated, resulting in Zn(CQ)2 neutral complexes, to which additional water molecules may coordinate. In the present work, the coordinative properties of clioquinol in aqueous solution have been analyzed by means of static, minimal cluster based DFT calculations and explicit solvent ab initio molecular dynamics simulations. Results from static calculations accounting for solvent effects by means of polarized continuum models suggest that the preferred metal coordination environment is tetrahedral Zn(CQ)2, whereas ab initio molecular dynamics simulations point to quasi degenerate penta Zn(CQ)2(H2O) and hexa Zn(CQ)2(H2O)2 coordinated complexes. The possible reasons for these discrepant results are discussed. PMID:25939963
Jiménez-Hoyos, Carlos A; Scuseria, Gustavo E
2014-01-01
Most {\\em ab initio} calculations on fullerene molecules have been carried out based on the paradigm of the H\\"uckel model. This is consistent with the restricted nature of the independent-particle model underlying such calculations, even in single-reference-based correlated approaches. On the other hand, previous works on some of these molecules using model Hamiltonians have clearly indicated the importance of short-range inter-atomic spin-spin correlations. In this work, we consider {\\em ab initio} non-collinear Hartree--Fock (HF) solutions for representative fullerene systems: the bowl, cage, ring, and pentagon isomers of C$_{20}$, and the larger C$_{30}$, C$_{36}$, C$_{60}$, C$_{70}$, and C$_{84}$ fullerene cages. In all cases but the ring we find that the HF minimum corresponds to a truly non-collinear solution with a torsional spin density wave. Optimized geometries at the generalized HF (GHF) level lead to fully symmetric structures, even in those cases where Jahn-Teller distortions have been previousl...
Pyrophyllite, Al2[Si4O10](OH)2, is the simplest structural prototype for 2:1 dioctahedral phyllosilicate. Because the net electric charge in pyrophyllite is zero, it is the best candidate for investigating the non electrostatic contribution to sorption and transport phenomena in clays. Using ab-initio simulations, we have investigated the reactivity and structure of the water-solid interface on the basal plane and edge sites of pyrophyllite. The calculations predict slightly hydrophobic behaviour of the basal plane. For the high water coverage (100), (110) and (-110), lateral facets have a lower energy than for the (010), (130) and (-130) surfaces. Analysis of the surface reactivity reveals that the =Al-OH groups are most easily protonated on the (010), (130) and (-130) facets. The =Al-O-Si= sites will be protonated on the (100), (130), (110), (-110) and (-130) surfaces. The =Al-OH2 complexes are more easily de-protonated than the =Si-OH and =Al-OH sites. A spontaneous, reversible exchange of the protons between the solution and the edge sites has been observed in ab-initio molecular dynamics simulations at 300 K. Such near-surface proton diffusion may result in a significant contribution to the diffusion coefficients measured in neutron scattering experiments. (author)
Sanson, Andrea; Giarola, Marco; Rossi, Barbara; Mariotto, Gino; Cazzanelli, Enzo; Speghini, Adolfo
2012-12-01
The vibrational properties of yttrium orthovanadate (YVO4) single crystals, with tetragonal zircon structure, have been investigated by means of polarized micro-Raman spectroscopy and ab initio calculations. Raman spectra were taken at different polarizations and orientations carefully set by the use of a micromanipulator, so that all of the twelve Raman-active modes, expected on the basis of the group theory, were selected in turn and definitively assigned in wave number and symmetry. In particular the Eg(4) mode, assigned incorrectly in previous literature, has been observed at 387 cm-1. Moreover, the very weak Eg(1) mode, peaked at about 137 cm-1, was clearly observed only under some excitation wavelengths, and its peculiar Raman excitation profile was measured within a wide region of the visible. Finally, ab initio calculations based on density-functional theory have been performed in order to determine both Raman and infrared vibrational modes and to corroborate the experimental results. The rather good agreement between computational and experimental frequencies is slightly better than in previous computational works and supports our experimental symmetry assignments.
Witczak, Przemysław; Kempisty, Pawel; Strak, Pawel [Institute of High Pressure Physics, Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw (Poland); Krukowski, Stanisław, E-mail: stach@unipress.waw.pl [Institute of High Pressure Physics, Polish Academy of Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland and Interdisciplinary Centre for Modelling, University of Warsaw, Pawińskiego 5a, 02-106 Warsaw (Poland)
2015-11-15
Ab initio studies of a GaN(0001)-Ga system with various thicknesses of a metallic Ga layer were undertaken. The studied systems extend from a GaN(0001) surface with a fractional coverage of gallium atoms to a Ga-GaN metal–semiconductor (m–s) contact. Electronic properties of the system are simulated using density functional theory calculations for different doping of the bulk semiconductor. It is shown that during transition from a bare GaN(0001) surface to a m–s heterostructure, the Fermi level stays pinned at a Ga-broken bond highly dispersive surface state to Ga–Ga states at the m–s interface. Adsorption of gallium leads to an energy gain of about 4 eV for a clean GaN(0001) surface and the energy decreases to 3.2 eV for a thickly Ga-covered surface. The transition to the m–s interface is observed. For a thick Ga overlayer such interface corresponds to a Schottky contact with a barrier equal to 0.9 and 0.6 eV for n- and p-type, respectively. Bond polarization-related dipole layer occurring due to an electron transfer to the metal leads to a potential energy jump of 1.5 eV, independent on the semiconductor doping. Additionally high electron density in the Ga–Ga bond region leads to an energy barrier about 1.2 eV high and 4 Å wide. This feature may adversely affect the conductivity of the n-type m–s system.
We present a comparative ab initio study of Li, Na, and Mg storage in tin, including phononic effects and phase competition between α and β Sn. Mg doping at low concentration is found to stabilize the β phase. On the contrary, Li and Na doping is shown to reverse the stability of the phases at room temperature: Li/Na-doped α-Sn is more stable than Li/Na-doped β-Sn up to a temperature of around 380/400 K. This may rationalize the formation of α-Sn upon lithiation and delithiation of β-Sn anodes reported in experimental studies. The changes in phase stability with Li/Na/Mg doping are directly related to the intercalation energies of Li/Na/Mg in one phase versus the other: at 300 K, Li/Na is easier intercalated in α-Sn (−0.37/−0.08 eV) than in β-Sn (0.06/0.49 eV), while Mg intercalation energy is, although positive (i.e., unfavored intercalation), lower in β-Sn (0.53 eV) than in α-Sn (0.66 eV). The temperature effect is found to affect significantly the intercalation energy, by up to 0.13 eV at 300 K. Analysis of diffusion barriers shows that Li, Na, and Mg diffusion in β-Sn is anisotropic with migration barriers along the (001) direction (respectively, 0.01, 0.22, and 0.07 eV) significantly lower than those in α-Sn (respectively, 0.20, 0.52, and 0.40 eV)
Legrain, F.; Manzhos, S., E-mail: mpemanzh@nus.edu.sg [Department of Mechanical Engineering, National University of Singapore, 117576 Singapore (Singapore); Malyi, O. I. [Centre for Materials Science and Nanotechnology, University of Oslo, NO-0316 Oslo (Norway); Persson, C. [Centre for Materials Science and Nanotechnology, University of Oslo, NO-0316 Oslo (Norway); Department of Physics, University of Oslo, NO-0316 Oslo (Norway)
2015-11-28
We present a comparative ab initio study of Li, Na, and Mg storage in tin, including phononic effects and phase competition between α and β Sn. Mg doping at low concentration is found to stabilize the β phase. On the contrary, Li and Na doping is shown to reverse the stability of the phases at room temperature: Li/Na-doped α-Sn is more stable than Li/Na-doped β-Sn up to a temperature of around 380/400 K. This may rationalize the formation of α-Sn upon lithiation and delithiation of β-Sn anodes reported in experimental studies. The changes in phase stability with Li/Na/Mg doping are directly related to the intercalation energies of Li/Na/Mg in one phase versus the other: at 300 K, Li/Na is easier intercalated in α-Sn (−0.37/−0.08 eV) than in β-Sn (0.06/0.49 eV), while Mg intercalation energy is, although positive (i.e., unfavored intercalation), lower in β-Sn (0.53 eV) than in α-Sn (0.66 eV). The temperature effect is found to affect significantly the intercalation energy, by up to 0.13 eV at 300 K. Analysis of diffusion barriers shows that Li, Na, and Mg diffusion in β-Sn is anisotropic with migration barriers along the (001) direction (respectively, 0.01, 0.22, and 0.07 eV) significantly lower than those in α-Sn (respectively, 0.20, 0.52, and 0.40 eV)
Ab Initio Nuclear Structure and Reaction Calculations for Rare Isotopes
We have developed a novel ab initio symmetry-adapted no-core shell model (SA-NCSM), which has opened the intermediate-mass region for ab initio investigations, thereby providing an opportunity for first-principle symmetry-guided applications to nuclear structure and reactions for nuclear isotopes from the lightest p-shell systems to intermediate-mass nuclei. This includes short-lived proton-rich nuclei on the path of X-ray burst nucleosynthesis and rare neutron-rich isotopes to be produced by the Facility for Rare Isotope Beams (FRIB). We have provided ab initio descriptions of high accuracy for low-lying (including collectivity-driven) states of isotopes of Li, He, Be, C, O, Ne, Mg, Al, and Si, and studied related strong- and weak-interaction driven reactions that are important, in astrophysics, for further understanding stellar evolution, X-ray bursts and triggering of s, p, and rp processes, and in applied physics, for electron and neutrino-nucleus scattering experiments as well as for fusion ignition at the National Ignition Facility (NIF).
Ab Initio Nuclear Structure and Reaction Calculations for Rare Isotopes
Draayer, Jerry P. [Louisiana State Univ., Baton Rouge, LA (United States)
2014-09-28
We have developed a novel ab initio symmetry-adapted no-core shell model (SA-NCSM), which has opened the intermediate-mass region for ab initio investigations, thereby providing an opportunity for first-principle symmetry-guided applications to nuclear structure and reactions for nuclear isotopes from the lightest p-shell systems to intermediate-mass nuclei. This includes short-lived proton-rich nuclei on the path of X-ray burst nucleosynthesis and rare neutron-rich isotopes to be produced by the Facility for Rare Isotope Beams (FRIB). We have provided ab initio descriptions of high accuracy for low-lying (including collectivity-driven) states of isotopes of Li, He, Be, C, O, Ne, Mg, Al, and Si, and studied related strong- and weak-interaction driven reactions that are important, in astrophysics, for further understanding stellar evolution, X-ray bursts and triggering of s, p, and rp processes, and in applied physics, for electron and neutrino-nucleus scattering experiments as well as for fusion ignition at the National Ignition Facility (NIF).
Homologous series of layered structures in binary and ternary Bi-Sb-Te-Se systems: Ab initio study
Govaerts, K.; Sluiter, M. H. F.; Partoens, B.; Lamoen, D.
2014-02-01
In order to account explicitly for the existence of long-periodic layered structures and the strong structural relaxations in the most common binary and ternary alloys of the Bi-Sb-Te-Se system, we have developed a one-dimensional cluster expansion (CE) based on first-principles electronic structure calculations, which accounts for the Bi and Sb bilayer formation. Excellent interlayer distances are obtained with a van der Waals density functional. It is shown that a CE solely based on pair interactions is sufficient to provide an accurate description of the ground-state energies of Bi-Sb-Te-Se binary and ternary systems without making the data set of ab initio calculated structures unreasonably large. For the binary alloys A1-xQx (A =Sb, Bi; Q =Te, Se), a ternary CE yields an almost continuous series of (meta)stable structures consisting of consecutive A bilayers next to consecutive A2Q3 for 00.6, the binary alloy segregates into pure Q and A2Q3. The Bi-Sb system is described by a quaternary CE and is found to be an ideal solid solution stabilized by entropic effects at T ≠0 K but with an ordered structure of alternating Bi and Sb layers for x =0.5 at T =0 K. A quintuple CE is used for the ternary Bi-Sb-Te system, where stable ternary layered compounds with an arbitrary stacking of Sb2Te3,Bi2Te3, and Te-Bi-Te-Sb-Te quintuple units are found, optionally separated by mixed Bi/Sb bilayers. Electronic properties of the stable compounds were studied taking spin-orbit coupling into account.
Marqués, M.; González, D. J.; González, L. E.
2016-07-01
The melting curve of sodium for a pressure range up to 100 GPa has been evaluated by the orbital free ab initio molecular dynamics method. This method uses the electronic density as the basic variable combined with an approximate electronic kinetic energy functional and a local ionic pseudopotential and makes it possible to perform simulations with a large number of particles and for long simulation times. The calculated melting curve shows a maximum melting temperature at a pressure around 30 GPa followed by a steep decrease up to 100 GPa. For various pressures and temperatures we have evaluated several static properties, including average and local structure, electronic properties, like the electron localization function (ELF), and dynamic properties, both single-particle and collective ones, from which some transport coefficients are deduced. Despite the accurate reproduction of the available experimental data, we do not observe any indication of an early transition from a bcc-like to an fcc-like liquid, as suggested previously by other authors, but rather pressure-induced change in the variation of icosahedral-like order and bcc-like order, with no sign of fcc-like structures in the whole liquid range studied. We also consider the evolution of the ELF within this type of local arrangement upon pressurization. In the dynamic realm, we find an enlarged wave-vector region where atomic collisions play an important role in the dynamic properties of the system as pressure is increased and temperature decreased along the melting line, leading to a peculiar behavior of the dynamic properties.
Chandel, Surjeet Kumar; Ahluwalia, P. K.; Sharma, Raman [Department of Physics, Himachal Pradesh University, Shimla, Himachal Pradesh-171005 (India); Kumar, Arun, E-mail: arun242493@yahoo.com [Department of Physics, Govt. College Banjar, Kullu, Himachal Pradesh-175123 (India)
2015-06-24
First principle calculations based on DFT have been performed to study the interaction of monoatomically thin Cu wire with silicon nanotube in armchair configuration having chirality (6, 6) both by placing it inside (encapsulation) and outside (functionalisation) the tube. The lowest energy for positioning monoatomically thin Cu wire inside and outside surfaces of SiNT were found to possess cohesive energies of 4.03 eV and 4.02 eV respectively and hence the stability of both SiNTs is found to be almost same. However, From the electronic band structures study, the conductance in case of SiNT for the encapsulated and functionalized positioning of the Cu wire have been found to be 2G{sub 0} and 4G{sub 0} respectively showing enhanced conductance for the functionalized SiNT.
First principle calculations based on DFT have been performed to study the interaction of monoatomically thin Cu wire with silicon nanotube in armchair configuration having chirality (6, 6) both by placing it inside (encapsulation) and outside (functionalisation) the tube. The lowest energy for positioning monoatomically thin Cu wire inside and outside surfaces of SiNT were found to possess cohesive energies of 4.03 eV and 4.02 eV respectively and hence the stability of both SiNTs is found to be almost same. However, From the electronic band structures study, the conductance in case of SiNT for the encapsulated and functionalized positioning of the Cu wire have been found to be 2G0 and 4G0 respectively showing enhanced conductance for the functionalized SiNT
Gonçalves, J N; Correia, J G; Butz, T; Picozzi, S; Fenta, A S; Amaral, V S
2012-01-01
The hyperfine interaction between the quadrupole moment of atomic nuclei and the electric field gradient (EFG) provides information on the electronic charge distribution close to a given atomic site. In ferroelectric materials, the loss of inversion symmetry of the electronic charge distribution is necessary for the appearance of the electric polarization. We present first-principles density functional theory calculations of ferroelectrics such as BaTiO$_{3}$, KNbO$_{3}$, PbTiO$_{3}$ and other oxides with perovskite structures, by focusing on both EFG tensors and polarization. We analyze the EFG tensor properties such as orientation and correlation between components and their relation with electric polarization. This work supports previous studies of ferroelectric materials where a relation between EFG tensors and polarization was observed, which may be exploited to study the ferroelectric order when standard techniques to measure polarization are not easily applied.
Pan, Hui
2014-01-01
Design of novel materials for efficient solar energy conversion is critical to the development of green energy technology. In this work, we present a first-principles study on the design of nanostructures for solar energy harvesting on the basis of the density functional theory. We show that the indirect band structure of bulk silicon nitride is transferred to direct bandgap in nanowire. We find that intermediate bands can be created by doping, leading to enhancement of sunlight absorption. W...
Pressure induced structural phase transition in SnS—An ab initio study
M Rajagopalan; G Kalpana; V Priyamvadha
2006-02-01
The structural behaviour of SnS under pressure has been investigated by first principle density functional calculations of the total energy by the TB–LMTO approach. We find that SnS undergoes a structural phase transition from orthorhombic type to monoclinic type structure around 17 GPa which is in good agreement with the recent experimental study. In addition, the ground state properties are computed and compared with the available results.
On the diverse bonding situations in nanostructures : an ab initio computational study
Pankewitz, Tobias
2010-01-01
This computational study investigates diverse bonding situations in nanostructures (carbon nanotubes, fullerenes, metal compounds) spanning a broad range of energies. Weak, dispersive interactions and covalent metal-ligand and metal-metal bonding are examined. The results of efficient density functional calculations are compared to those of correlated wavefunction calculations on model systems. This rigorous validation is crucial in evaluating the balance between computational cost and accuracy.
Hydrogen Diffusion in Silicon - An ab initio Study of Hydrogen Kinetic Properties in Silicon
Bilteanu, Liviu; Posselt, Mathias; Crocombette, Jean-Paul
2011-01-01
In this paper we present kinetic properties such as migration and decomposition barriers of hydrogen defects in silicon calculated by Density Functional Theory (DFT) based methods. We study the following defects: H atoms (H) and ions (H+, H-), hydrogen molecules (H2) and hydrogen complexes (H2*). Our results show that the dominating migration species are H ions, their charge-independent migration barrier being 0.46 eV in excellent agreement with experimental value [1]. At higher temperatures ...
Magnetism in Sr2CrMoO6 : A combined ab initio and model study
Sanyal, Prabuddha; Halder, Anita; Si, Liang; Wallerberger, Markus; Held, Karsten; Saha-Dasgupta, Tanusri
2016-07-01
Using a combination of first-principles density functional theory (DFT) calculations and exact diagonalization studies of a first-principles derived model, we carry out a microscopic analysis of the magnetic properties of the half-metallic double perovskite compound Sr2CrMoO6 , a sister compound of the much discussed material Sr2FeMoO6 . The electronic structure of Sr2CrMoO6 , though appearing similar to Sr2FeMoO6 at first glance, shows nontrivial differences with that of Sr2FeMoO6 on closer examination. In this context, our study highlights the importance of charge transfer energy between the two transition metal sites. The change in charge transfer energy due to a shift of Cr d states in Sr2CrMoO6 compared to Fe d in Sr2FeMoO6 suppresses the hybridization between Cr t2 g and Mo t2 g. This strongly weakens the hybridization-driven mechanism of magnetism discussed for Sr2FeMoO6 . Our study reveals that, nonetheless, the magnetic transition temperature of Sr2CrMoO6 remains high since an additional superexchange contribution to magnetism arises with a finite intrinsic moment developed at the Mo site. We further discuss the situation in comparison to another related double perovskite compound, Sr2CrWO6 . We also examine the effect of correlation beyond DFT, using dynamical mean field theory.
Ab-initio study on the possible doping strategies for MoS$_2$ monolayers
Dolui, Kapildeb; Rungger, Ivan; Pemmaraju, Chaitanya Das; Sanvito, Stefano
2013-01-01
Density functional theory is used to systematically study the electronic and magnetic properties of doped MoS$_2$ monolayers, where the dopants are incorporated both via S/Mo substitution or as adsorbates. Among the possible substitutional dopants at the Mo site, Nb is identified as suitable p-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS$_2$ conduction band, ...
Ab-Initio Study of Magnetic Properties of Mn-doped MgSiN
Rufinus, Jeffrey
2010-03-01
The current interest in the field of semiconductor spintronics is mostly focused on transition metal-doped binary materials. Recently, however, the explorations of transition metal-dopd ternary semiconductors have gained attention, duel to experimental confirmations of possible high Curie temperature in chalcopyrite compounds. A density functional theory study was performed on Mn-doped ternary material MgSiN2. Our results show Mn-doped MgSiN2 to be antiferromagnetic for MnMg (Mn substitutes Mg site) and ferromagnetic for MnSi (Mn substitutes Si site).
Raman and ab initio studies of simple and binary 1-alkyl-3-methylimidazolium ionic liquids
Berg, R.W.; Deetlefs, M.; Seddon, K.R.; Shim, Irene; Thompson, J.M.
2005-01-01
calculations. The previously reported anti and gauche forms of the [C(4)mim](+) cation have been observed, and this study reveals this to be a general feature of the long-chain I-alkyl derivatives. Analysis of mixtures Of [C(6)mim]Cl and [C(6)mim][PF6] has provided information on the nature of the hydrogen...... bonding between the imidazolium headgroup and the anions, and the invariance of the essentially 50:50 mixture of the predominant conformers informs on the nature of glass formation in these systems....
Ab initio study of double perovskites Ba2DySbO6
Jha, Dhiraj Kumar; Mandal, Golak; Ray, Chandan; Himanshu, A. K.; Singh, B. K.; Kumar, Uday; Choudhary, B. K.
2016-05-01
First principle study of the electronic band structure of Ba2DySbO6 synthesied by the solid state reaction technique have been performed within the framework of density function theory using WIEN2K. It has been shown in the absence of electron-electron interaction (U=0), BaDySO6 behaves like a half-metal. Even in the presence of DFT+U, electron-electron interaction via the Hubbard term (from U = 0, 2.72e -7.02eV), it still shows half metals.
Electronic and thermal properties of TiFe{sub 2} compound: An ab initio study
Sathyakumari, V. S.; Sankar, S., E-mail: drshreemit@gmail.com; Mahalakshmi, K.; Subashree, G.; Krithiga, R. [Condensed Matter Laboratory, Department of Physics, Madras Institute of Technology Campus, Anna University, Chennai-600044, Tamilnadu (India)
2015-06-24
A systematic study of electronic, and thermal properties such as the Density of states, Fermi energy, Debye temperature and specific heat coefficient, has been carried out using the results of electronic bandstructure and related characteristics of the Laves phase compound, TiFe{sub 2}. Computation of electronic bandstructure and associated properties has been carried out using the tight-binding-linear-muffin-tin-orbital (TB-LMTO) method within atomic sphere approximation (ASA). The calculated values are compared with the available results of literature.
An ab initio study of WO3 under pressure up to 30 GPa
High-pressure polymorphs of WO3 have been studied with a first-principles pseudopotential method. The medium-range (0.01-20 GPa) and high-range (20-30 GPa) polymorphs have been characterized and are compared with recent experimental results. The main new feature is the appearance of a sevenfold coordinated tungsten in the high-pressure polymorph. The subtle phase transitions that were induced from Raman spectra evolutions have not been confirmed. However, changes in the W-O distances and O-W-O and W-O-W angles may explain the changes in Raman spectra
Equation of State for Shock Compressed Xenon in the Ionization Regime：ab Initio Study
王聪; 顾云军; 陈其峰; 贺贤土; 张平
2012-01-01
Quantum molecular dynamic （QMD） simulations have been applied to study the thermophysical properties of liquid xenon under dynamic compressions. The equation of state （EOS） obtained from QMD calculations are corrected according to Saha equation, and contributions from atomic ionization, which are of predominance in determining the EOS at high temperature and pressure, are considered. For the pressures below 160 GPa, the necessity in accounting for the atomic ionization has been demonstrated by the Hugoniot curve, which shows excellent agreement with previous experimental measurements, and three levels of ionization have been proved to be sufficient at this stage.
Ab initio electronic structure study of a model water splitting dimer complex.
Fernando, Amendra; Aikens, Christine M
2015-12-28
A model manganese dimer electrocatalyst bridged by μ-OH ligands is used to investigate changes in spin states that may occur during water oxidation. We have employed restricted open-shell Hartree-Fock (ROHF), second-order Møller-Plesset perturbation theory (MP2), complete active space self-consistent field (CASSCF), and multireference second-order Møller-Plesset perturbation theory (MRMP2) calculations to investigate this system. Multiconfigurational methods like CASSCF and MRMP2 are appropriate methods to study these systems with antiferromagnetically-coupled electrons. Orbital occupations and distributions have been closely analyzed to understand the electronic details and contributions to the water splitting from manganese and oxygen atoms. The presence of Mn(IV)O˙ radical moieties has been observed in this catalytic pathway. Multiple nearly degenerate excited states were found close to the ground state in all structures. This suggests competing potential energy landscapes near the ground state may influence the reactivity of manganese complexes such as the dimers studied in this work. PMID:26593689
Ab initio study of the trapping of polonium on noble metals
Rijpstra, Kim; Van Yperen-De Deyne, Andy; Maugeri, Emilio Andrea; Neuhausen, Jörg; Waroquier, Michel; Van Speybroeck, Veronique; Cottenier, Stefaan
2016-04-01
In the future MYRRHA reactor, lead bismuth eutectic (LBE) will be used both as coolant and as spallation target. Due to the high neutron flux a small fraction of the bismuth will transmute to radiotoxic 210Po. Part of this radiotoxic element will evaporate into the gas above the coolant. Extracting it from the gas phase is necessary to ensure a safe handling of the reactor. An issue in the development of suitable filters is the lack of accurate knowledge on the chemical interaction between a candidate filter material and either elemental polonium or polonium containing molecules. Experimental work on this topic is complicated by the high radiotoxicity of polonium. Therefore, we present in this paper a first-principles study on the adsorption of polonium on noble metals as filter materials. The adsorption of monoatomic Po is considered on the candidate filter materials palladium, platinum, silver and gold. The case of the gold filter is looked upon in more detail by examining how bismuth pollution affects its capability to capture polonium and by studying the adsorption of the heavy diatomic molecules Po2, PoBi and PoPb on this gold filter.
Fermiology of 122 family of Fe-based superconductors: An ab initio study
Fermiology of various 122 systems are studied through first principles simulation. Electron doping causes expansion of electron and shrinkage of hole Fermi pockets. Isovalent Ru substitution (up to 35%) makes no visible modification in the electron- and hole-like Fermi surfaces (FSs) providing no clue regarding the nature of charge carrier doping. However, in case of 32% P doping there are considerable changes in the hole FSs. From our calculations, it is very clear that two-dimensionality of FSs may favour electron pair scattering between quasi-nested FSs which has important bearings in various orders (magnetic, orbital, superconducting) present in Fe-based superconductors. - Highlights: • DFT-based simulated Fermi surfaces of 122 family of Fe-based superconductors are studied. • Room-temperature experimental structural parameters are used as input of our calculations. • Topological changes in the FS structures for various kinds of doping are presented. • Influence of dimensional cross-over of FS structures in magnetism and superconductivity is investigated
Ab initio molecular orbital study on the Ge-, Sn-, Zr- and Si/Ge-mixed silsesquioxanes.
Kudo, Takako; Akasaka, Mitsutoshi; Gordon, Mark S
2008-05-29
For the purpose of designing new functional silsesquioxanes (POSS), the structure and stability of the analogous compounds of the heavier group 14 and 4 elements such as germanium (Ge-POSS), tin (Sn-POSS) and zirconium (Zr-POSS) analogues of POSS were investigated and compared with those of the parent POSS and the titanium analogue (Ti-POSS) with electronic structure theory calculations, including electron correlation effects. In order to obtain information about the metalloxane (-X-O-X-) linkage, the structures and properties of the building blocks of metallasilsesquioxanes, such as dimetalloxanes, H(OH)2XOX(OH)2H, X = Ge, Sn and Zr, and cyclometalloxanes, [H(OH)XO]n , n = 3-6, X = Ge, Sn and Zr, were examined. The stability of the Si/Ge-mixed POSS were also studied in comparison with POSS and the completely germanium-substituted POSS. PMID:18444638
Carbon- and silicon-capped silicon carbide nanotubes: An ab initio study
Adhikari, K.; Ray, A. K.
2011-04-01
A systematic study of fullerene hemisphere capped finite SiC nanotubes is presented. The tubes are spin optimized using the hybrid functional B3LYP (Becke's three-parameter exchange and the Lee-Yang-Parr correlation functionals) and an all electron 3-21G * basis. Capping of a SiC nanotube changes cohesive energy, HOMO-LUMO gap and other electronic and geometric properties of a SiC nanotube. Also, the carbon-capped SiC nanotubes are energetically preferable compared to silicon-capped tubes. For example, the binding energy per atom for hydrogen-terminated “infinite” SiC nanotube (5,5) having five unit cells is 4.993 eV, the corresponding numbers being 5.989 eV and 4.812 eV for C-capped and Si-capped nanotubes, respectively.
Absorption and adsorption of hydrogen in B2-FeAl: Ab initio study
Gallouze, M. [Laboratoire de Physique Théorique, Faculté de Physique, USTHB, BP 32 El Alia, 16111 Bab Ezzouar, Alger (Algeria); Kellou, A., E-mail: afkellou@usthb.dz [Laboratoire de Physique Théorique, Faculté de Physique, USTHB, BP 32 El Alia, 16111 Bab Ezzouar, Alger (Algeria); Hamoutene, D. [Laboratoire de Modélisation Thermodynamique et Moléculaire, Faculté de Chimie, USTHB, BP 32 El Alia, 16111 Bab Ezzouar, Alger (Algeria); Grosdidier, T. [Laboratoire d' Etudes des Microtextures et de Mécanique des Matériaux, UMR CNRS 7239, Ile du Saulcy, 57045 Metz Cedex 01 (France); Drir, M. [Laboratoire de Physique Théorique, Faculté de Physique, USTHB, BP 32 El Alia, 16111 Bab Ezzouar, Alger (Algeria)
2013-05-01
Using the density functional theory and the pseudo-potential approach, the behavior of atomic hydrogen in bulk and surface of B2-FeAl was studied. In Al-rich environment without structural defects, the hydrogen atom prefers to stabilize in octahedral site. However, the most favorable double defect cases were hydrogen with Al-vacancy followed by hydrogen with Al-antisite. For the surface case, the obtained results have shown that H was always attracted by (0 0 1) and (1 1 0) surfaces. The diffusion of H to the bulk was predicted to be favorable for (1 1 0) and unfavorable for (0 0 1) Fe-terminated. The behavior of H in B2-FeAl H was highly dependent on the local environment and the surface orientation.
Ab initio study of metastability of Eu3+ defect complexes in GaN
Density functional theory (DFT) within the generalized gradient approximation (GGA) has been used to study the structural and electronic properties of Eu3+ defect complexes in GaN under Ga-rich conditions. Two distinct configurations of the EuGaVN defect complex, the axial and basal configuration, have been investigated. We report two forms of metastable defects namely; the Negative U defect in the lower half of the GaN band-gap and a metastable defect with two distinct configurations each with levels at EC −0.46 eV and −0.56 eV in the upper half of the GaN band-gap.
Arindam Bankura; Amalendu Chandra
2005-10-01
The hydration structure and translocation of an excess proton in hydrogen bonded water clusters of two different sizes are investigated by means of finite temperature quantum simulations. The simulations are performed by employing the method of Car–Parrinello molecular dynamics where the forces on the nuclei are obtained directly from `on the fly' quantum electronic structure calculations. Since no predefined interaction potentials are used in this scheme, it is ideally suited to study proton translocation processes which proceed through breaking and formation of chemical bonds. The coordination number of the hydrated proton and the index of oxygen to which the excess proton is attached are calculated along the simulation trajectories for both the clusters.
Experimental and ab initio studies of the novel piperidine-containing acetylene glycols
Mirsakiyeva, Amina; Elgammal, Karim; Ten, Assel; Hugosson, Håkan W; Delin, Anna; Yu, Valentina K
2015-01-01
Synthesis routes of novel piperidine-containing diacetylene are presented. The new molecules are expected to exhibit plant growth stimulation properties. In particular, the yield in a situation of drought is expected to increase. The synthesis makes use of the Favorskii reaction between cycloketones/piperidone and triple-bond containing glycols. The geometries of the obtained molecules were determined using nuclear magnetic resonance (NMR). The electronic structure and geometries of the molecules were studied theoretically using first-principles calculations based on density functional theory. The calculated geometries agree very well with the experimentally measured ones, and also allow us to determine bond lengths, angles and charge distributions inside the molecules. The stability of the OH-radicals located close to the triple bond and the piperidine/cyclohexane rings was proven by both experimental and theoretical analyses. The HOMO/LUMO analysis was done in order to characterize the electron density of t...
Ab initio studies of Co2FeAl1-xSix Heusler alloys
Szwacki, N. Gonzalez; Majewski, Jacek A.
2016-07-01
We present results of extensive theoretical studies of Co2FeAl1-xSix Heusler alloys, which have been performed in the framework of density functional theory employing the all-electron full-potential linearized augmented plane-wave scheme. It is shown that the Si-rich alloys are more resistive to structural disorder and as a consequence Si stabilizes the L21 structure. Si alloying changes position of the Fermi level, pushing it into the gap of the minority spin-band. It is also shown that the hyperfine field on Co nuclei increases with the Si concentration, and this increase originates mostly from the changes in the electronic density of the valence electrons.
Pentagonal dodecahedron methane hydrate cage and methanol system—An ab initio study
Snehanshu Pal; T K Kundu
2013-03-01
Density functional theory based studies have been performed to elucidate the role of methanol as an methane hydrate inhibitor. A methane hydrate pentagonal dodecahedron cage’s geometry optimization, natural bond orbital (NBO) analysis, Mullikan charge determination, electrostatic potential evaluation and vibrational frequency calculation with and without the presence of methanol using WB97XD/6-31++G(d,p) have been carried out. Calculated geometrical parameters and interaction energies indicate that methanol destabilizes pentagonal dodecahedron methane hydrate cage (1CH4@512) with and without the presence of sodium ion. NBO analysis and red shift of vibrational frequency reveal that hydrogen bond formation between methanol and water molecules of 1CH4@512 cage is favourable subsequently after breaking its original hydrogen bonded network.
Ab initio study of native defects in SnO under strain
Granato, Danilo B.
2014-04-01
Tin monoxide (SnO) has promising properties to be applied as a p-type semiconductor in transparent electronics. To this end, it is necessary to understand the behaviour of defects in order to control them. We use density functional theory to study native defects of SnO under tensile and compressive strain. We show that Sn vacancies are less stable under tension and more stable under compression, irrespectively of the charge state. In contrast, O vacancies behave differently for different charge states. It turns out that the most stable defect under compression is the +1 charged O vacancy in an Sn-rich environment and the charge neutral O interstitial in an O-rich environment. Therefore, compression can be used to transform SnO from a p-type into either an n-type or an undoped semiconductor. Copyright © EPLA, 2014.
Possible doping strategies for MoS 2 monolayers: An ab initio study
Dolui, Kapildeb
2013-08-14
Density functional theory is used to systematically study the electronic properties of doped MoS2 monolayers, where the dopants are incorporated both via S/Mo substitution or as adsorbates. Among the possible substitutional dopants at the Mo site, Nb is identified as suitable p-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS2 conduction band, making the system n type. Finally, the adsorption of charged molecules is considered, mimicking an ionic liquid environment. We find that molecules adsorption can lead to both n- and p-type conductivity, depending on the charge polarity of the adsorbed species. © 2013 American Physical Society.
Photoionization of multishell fullerenes studied by ab initio and model approaches
Verkhovtsev, Alexey; Solov'yov, Andrey V
2016-01-01
Photoionization of two buckyonions, C$_{60}$@C$_{240}$ and C$_{20}$@C$_{60}$, is investigated by means of time-dependent density-functional theory (TDDFT). The TDDFT-based photoabsorption spectrum of C$_{60}$@C$_{240}$, calculated in a broad photon energy range, resembles the sum of spectra of the two isolated fullerenes, thus illustrating the absence of strong plasmonic coupling between the fullerenes which was proposed earlier. The calculated spectrum of the smaller buckyonion, C$_{20}$@C$_{60}$, differs significantly from the sum of the cross sections of the individual fullerenes because of strong geometrical distortion of the system. The contribution of collective electron excitations arising in individual fullerenes is evaluated by means of plasmon resonance approximation (PRA). An extension of the PRA formalism is presented, which allows for the study of collective electron excitations in multishell fullerenes under photon impact. An advanced analysis of photoionization of buckyonions, performed using m...
The spectroscopic constants and anharmonic force field of AgSH: An ab initio study.
Zhao, Yanliang; Wang, Meishan; Yang, Chuanlu; Ma, Xiaoguang; Zhu, Ziliang
2016-07-01
The equilibrium structure, spectroscopy constants, and anharmonic force field of silver hydrosulfide (AgSH) have been calculated at B3P86, B3PW91 and MP2 methods employing two basis sets, TZP and QZP, respectively. The calculated geometries, ground state rotational constants, harmonic vibrational wave numbers, and quartic and sextic centrifugal distortion constants are compared with the available experimental and theoretical data. The equilibrium rotational constants, fundamental frequencies, anharmonic constants, and vibration-rotation interaction constants, Coriolis coupling constants, cubic and quartic force constants are predicted. The calculated results show that the MP2/TZP results are in good agreement with experiment observation and are also an advisable choice to study the anharmonic force field of AgSH. PMID:27085293
AB INITIO STUDY OF ADVANCED METALLIC NUCLEAR FUELS FOR FAST BREEDER REACTORS
Landa, A; Soderlind, P; Grabowski, B; Turchi, P A; Ruban, A V; Vitos, L
2012-04-23
Density-functional formalism is applied to study the ground state properties of {gamma}-U-Zr and {gamma}-U-Mo solid solutions. Calculated heats of formation are compared with CALPHAD assessments. We discuss how the heat of formation in both alloys correlates with the charge transfer between the alloy components. The decomposition curves for {gamma}-based U-Zr and U-Mo solid solutions are derived from Ising-type Monte Carlo simulations. We explore the idea of stabilization of the {delta}-UZr{sub 2} compound against the {alpha}-Zr (hcp) structure due to increase of Zr d-band occupancy by the addition of U to Zr. We discuss how the specific behavior of the electronic density of states in the vicinity of the Fermi level promotes the stabilization of the U{sub 2}Mo compound. The mechanism of possible Am redistribution in the U-Zr and U-Mo fuels is also discussed.
Effects of additives in α- and θ-alumina: an ab initio study
It is of high fundamental and practical importance to be able to control the formation and stability of the different crystalline phases of alumina (Al2O3). In this study, we have used density functional theory methods to investigate the changes induced in the thermodynamically stable α phase and the metastable θ phase as one eighth of the Al atoms are substituted for different additives (Sc, W, Mo, Cr, Cu, Si, and B). The calculations predict that the additives strongly affect the relative stability between the two phases. Most tested additives are shown to shift the relative stability towards, and in some cases completely stabilize, the θ phase, while Cu doping is predicted to increase the relative stability of the α phase. The reasons for these effects are discussed, as are possible implications on the growth and use of doped aluminas in practical applications. In addition, the effects of the additives on bulk moduli and densities of states have been investigated
Pan, Hui
2014-01-01
Design of novel materials for efficient solar energy conversion is critical to the development of green energy technology. In this work, we present a first-principles study on the design of nanostructures for solar energy harvesting on the basis of the density functional theory. We show that the indirect band structure of bulk silicon nitride is transferred to direct bandgap in nanowire. We find that intermediate bands can be created by doping, leading to enhancement of sunlight absorption. We further show that codoping not only reduces the bandgap and introduces intermediate bands but also enhances the solubility of dopants in silicon nitride nanowires due to reduced formation energy of substitution. Importantly, the codoped nanowire is ferromagnetic, leading to the improvement of carrier mobility. The silicon nitride nanowires with direct bandgap, intermediate bands, and ferromagnetism may be applicable to solar energy harvesting. PMID:25294975
Ab Initio Study of the Dielectric and Electronic Properties of Multilayer GaS Films.
Li, Yan; Chen, Hui; Huang, Le; Li, Jingbo
2015-03-19
The dielectric properties of multilayer GaS films have been investigated using a Berry phase method and a density functional perturbation theory approach. A linear relationship has been observed between the number of GaS layers and slab polarizability, which can be easily converged at a small supercell size and has a weak correlation with different stacking orders. Moreover, the intercoupling effect of the stacking pattern and applied vertical field on the electronic properties of GaS bilayers has been discussed. The band gaps of different stacking orders show various downward trends with the increasing field, which is interpreted as giant Stark effect. Our study demonstrates that the slab polarizability as the substitution of conventional dielectric constant can act as an independent and reliable parameter to elucidate the dielectric properties of low-dimensional systems and that the applied electric field is an effective method to modulate the electric properties of nanostructures. PMID:26262870
Alkali earth metal silicides MSi (M = Mg, Ca, Sr, Ba) are multi-phase compound and exist simultaneously in CrB, CsCl, NaCl or rock salt (RS) and zinc blende (zb) structures. In the CrB and CsCl phases, their behavior is metallic in the non-magnetic (NM) as well as the ferromagnetic (FM) structure. The total spin magnetic moment of these compounds in the zb phase is more than that in the corresponding RS phase; therefore, detailed studies for the zb phase are presented in this paper. This study includes structural, electronic and mechanical properties by using the full potential linear augmented plain wave scheme with local orbitals. Ferromagnetic CaSi, SrSi and BaSi show true half-metallic character. For a better understanding of the half-metallicity in the above-mentioned sp-type compounds, their band structures have been calculated and densities of state plots have been produced. The FM structures are more stable and harder than the corresponding NM structures. The magnetic moment corresponding to equilibrium lattice constants is calculated as 2 µB for FM CaSi, SrSi and BaSi, which are in accordance with the earlier work on the sp-type compounds CaC, SrC and BaC. The FM character changes to the paramagnetic character as the lattice parameter decreases. The general trend is that the values of the elastic constants C11, C12 and C44 increase with increasing hydrostatic pressure
Ab Initio Study of the Electron Transfer in an Ionized Stacked Complex of Guanines
Cauët, Emilie; Liévin, Jacques
2009-08-01
The charge transfer process in an ionized stacking of two consecutive guanines (G5'G3')+ has been studied by means of state-averaged CASSCF/MRCI and RASSCF/RASPT2 calculations. The ground and two first excited states of the radical cation have been characterized, and the topology of the corresponding potential energy surfaces (PESs) has been studied as a function of all intermolecular geometrical parameters. The results demonstrate that the charge transfer process in (G5'G3')+ is governed by the avoiding crossing between the ground and first excited states of the complex. Relative translation motions of both guanines in their molecular planes are shown to lead to the charge migration between G5' and G3'. Five stationary points (three minima and two saddle points) have been characterized along the reaction path describing the passage of the positive charge from G5' to G3'. The global minimum on the PES is found to correspond to the charge configuration G5'+G3'. The existence of an intermediate minimum along the reaction path has been established, characterizing a structure where the positive charge is equally distributed between the two guanines. The calculated energy profile allowed us to determine the height of the potential energy barrier (7.33 kcal/mol) and to evaluate the electronic coupling at a geometry close to the avoiding crossing (3.6 kcal/mol). Test calculations showed that the topology of the ground state PES of the complex GG+ is qualitatively conserved upon optimization of the intramolecular geometrical parameters of the stationary points.
Storing of molecular hydrogen in graphite cell. An ab initio study
Full text: Weakly-bounded hydrogen bonds are of fundamental importance in structural chemistry and biology, supramolecular chemistry and crystal engineering. Molecular hydrogen (H2) holds a promise of the ideal energy source for production of electricity in transportable devices. Carbon-based materials, such as nano-structured graphite platelets (graphene) or cells are among the most attractive physisorption substrates. A computationally tractable model of graphene is provided by polycyclic aromatic hydrocarbons (PAH's) such as benzene. The importance of benzene and molecular hydrogen complex as a model system for molecular hydrogen storage in graphite cells as well as the importance of noncovalent interactions in a variety of different systems prompted us to study the H2-π interaction. Adequate quantum-chemical description of H2-PAH interactions require post-Hartree-Fock treatment of electron correlation and of basis set superposition error (BSSE) effects. In a previous study we analyzed the weakly-bounded hydrogen bonds between acetylene and different proton donors as a model of C-H-π interaction. Important BSSE effects have been found due to the long range interaction of electron correlation, especially for the dispersion energy component. Similar effects can be found in case of benzene and molecular hydrogen complex. To treat this effect, we use the well-known a posteriori counterpoise (CP) BSSE correction scheme introduced by Boys and Bernardi. The theoretical values of interaction energies between the benzene ring and the molecular hydrogen as well as the intermolecular distances between them show an important BSSE and basis size effects. (author)
Molina-Sanchez, Alejandro; Palummo, Maurizia; Marini, Andrea; Wirtz, Ludger
2015-03-01
Research on ultra-thin two-dimensional materials has been booming since the discovery of graphene along with its interesting physical properties. The transition metal dichalcogenides as MoSare gaining considerable attention due to their potential application in photovoltaics and nanoscale transistors. The optical properties of these layered materials depend strongly on the number of layers. The paradigmatic example is the transition from indirect to direct bandgap when we change from multi-layer to single-layer MoS. In this work, we study the effects of the electron-phonon interaction on the optical properties of single-layer MoS. In the framework of the GW method we calculate the contribution of the electron-phonon coupling to the self-energy. This allows us to calculate the zero-point re-normalization of the quasi-particle energies and to include temperature effects. We discuss the bandgap dependence on the temperature, and the change in the linewidth of the quasi-particle states. The impact of temperature on the exciton states is also addressed.
Ab initio study of structural stability of InAs nanowires
We have studied the structural stability of InAs nanowires with different diameters by using density functional theory - pseudopotential computations. The stable structure of bulk InAs is Zinc Blende (ZB) although under special experimental conditions Wurtzite (WZ) InAs has also been observed. Due to the high symmetry and low ratio of dangling bonds, we have considered hexagonal and triangular WZ nanowires in[0001] direction and hexagonal ZB nanowires in[111] direction. In order to select facets of the nanowires, we calculated formation energy of nonpolar surfaces of both structures and found that ZB(110) and WZ(10 anti 10) surfaces are more stable than others. All nanowires were calculated in the optimized and relaxed supercells. Calculating the cohesive energy of nanowires with small diameters, we found WZ structure more stable than ZB, in good agreement with experimental results. The obtained cohesive energy of the small diameter nanowires were fitted by a phenomenological model to obtain dangling bond energies and then extrapolate the cohesive energy of large diameter nanowires. The extrapolated results indicate that for diameters up to 50 Angstrom, the WZ nanowires are more stable than ZB nanowires while for larger diameters, the contribution of dangling bonds reduces and the results converge to that of bulk InAs.
Ab Initio Molecular Dynamics Study of Aqueous Solvation of Ethanol and Ethylene
Van Erp, T S; Erp, Titus S. van; Meijer, Evert Jan
2002-01-01
The structure and dynamics of aqueous solvation of ethanol and ethylene are studied by DFT-based Car-Parrinello molecular dynamics. We did not find an enhancement of the structure of the hydrogen bonded network of hydrating water molecules. Both ethanol and ethylene can easily be accommodated in the hydrogen-bonded network of water molecules without altering its structure. This is supports the conclusion from recent neutron diffraction experiments that there is no hydrophobic hydration around small hydrophobic groups. Analysis of the electronic charge distribution using Wannier functions shows that the dipole moment of ethanol increases from 1.8 D to 3.1 D upon solvation, while the apolar ethylene molecule attains an average dipole moment of 0.5 D. For ethylene, we identified configurations with $\\pi$-H bonded water molecules, that have rare four-fold hydrogen-bonded water coordination, yielding instantaneous dipole moments of ethylene of up to 1 D. The results provide valuable information for the improvement...
Dixit, Vivek; Nandadasa, Chandani N.; Kim, Seong-Gon, E-mail: kimsg@ccs.msstate.edu [Department of Physics and Astronomy, Mississippi State University, Mississippi State, Mississippi 39762, USA and Center for Computational Sciences, Mississippi State University, Mississippi State, Mississippi 39762 (United States); Kim, Sungho [Center for Computational Sciences, Mississippi State University, Mississippi State, Mississippi 39762 (United States); Park, Jihoon; Hong, Yang-Ki [Department of Electrical and Computer Engineering and MINT Center, The University of Alabama, Tuscaloosa, Alabama 35487 (United States)
2015-11-28
The first-principles density functional theory has been used to study Ga/In-substituted strontium hexaferrite (SrFe{sub 12}O{sub 19}). Based on the calculation of the substitution energy of Ga and In in SrFe{sub 12}O{sub 19} and the formation probability analysis, we conclude that in SrFe{sub 12−x}Ga{sub x}O{sub 19} the substituted Ga atoms prefer to occupy the 12k, 2a, and 4f{sub 1} sites, while In atoms in SrFe{sub 12−x}In{sub x}O{sub 19} occupy the 12k, 4f{sub 2}, and 4f{sub 1} sites. We used the site occupation probabilities to calculate the magnetic properties of the substituted SrFe{sub 12}O{sub 19}. It was found that as the fraction of Ga atoms in SrFe{sub 12−x}Ga{sub x}O{sub 19} increases, the saturation magnetization (M{sub s}) as well as magnetic anisotropy energy (MAE) decrease, while the anisotropy field (H{sub a}) increases. In the case of SrFe{sub 12−x}In{sub x}O{sub 19}, M{sub s}, MAE, and H{sub a} decrease with an increase of the concentration of In atoms.
Z-contrast imaging and ab initio study on "d" superstructure in sedimentary dolomite
Shen, Zhizhang; Szlufarska, Izabela; Brown, Philip E; Xu, Huifang
2016-01-01
Nano-precipitates with tripled periodicity along the c-axis are observed in a Ca-rich dolomite sample from Proterozoic carbonate rocks with "molar tooth" structure. This observation is consistent with previous description of d reflections. High-angle annular dark-field STEM imaging (or Z-contrast imaging) that avoids dynamic diffraction as seen in electron diffraction and high-resolution TEM imaging modes, confirms that d reflections correspond to nanoscale precipitates aligned parallel to (001) of the host dolomite. The lamellae precipitates have a cation ordering sequence of Ca-Ca-Mg-Ca-Ca- Mg along the c direction resulting in a chemical composition of Ca0.67Mg0.33CO3. This superstructure is attributed to the extra or d reflections, thus is referred to as the d superstructure in this study. The structure can be simply described as interstratified calcite/dolomite. The crystal structure of the d superstructure calculated from density functional theory (DFT) has a space group of P31c and has a and c unit-cel...
Elastic, Electronic, Optical and Thermal Properties of Na2Po: An Ab Initio Study
Baki, N.; Eithiraj, R. D.; Khachai, H.; Khenata, R.; Murtaza, G.; Bouhemadou, A.; Seddik, T.; Bin-Omran, S.
2016-01-01
The structural, elastic, electronic, optical and thermodynamic properties of the sodium polonide Na2Po compound have been studied through the full potential linearized augmented plane wave plus local orbitals (FP-LAPW + lo) and tight-binding linear muffin-tin orbital (TB-LMTO) methods. The exchange-correlation potential was treated within the local density approximation for the TB-LMTO calculations and within the generalized gradient approximation for the FP-LAPW + lo calculations. In addition, Tran and Blaha-modified Becke-Johnson (TB-mBJ) potential and Engel-Vosko generalized gradient approximation were used for the electronic and optical properties. Ground state properties such as the equilibrium lattice constant, bulk modulus and its pressure derivative were calculated and compared with available data. The single-crystal and polycrystalline elastic constants of the considered compound were calculated via the total energy versus strain in the framework of the FP-LAPW + lo approach. The calculated electronic structure reveals that Na2Po is a direct band gap semiconductor. The frequency-dependent dielectric function, refractive index, extinction coefficient, reflectivity coefficient and electron energy loss function spectra are calculated for a wide energy range. The variations of the lattice constant, bulk modulus, heat capacity, volume expansion coefficient and Debye temperature with temperature and pressure were calculated successfully using the FP-LAPW + lo method in combination with the quasi-harmonic Debye model.
Ab initio study of pressure-induced magnetic transition in manganese pnictides
Prathiba, G.; Naanci, B. Anto; Rajagopalan, M.
2007-02-01
We report a density functional calculation on the NiAs-type Mn-based pnictides. The total energy as a function of volume is obtained by means of self-consistent tight-binding linear muffin-tin orbital method by performing spin and non-spin polarized calculation. From the present study, we predict a magnetic-phase transition from ferromagnetic (FM) to non-magnetic (NM) around 49 and 35.7 GPa for MnAs and MnSb, respectively. The pressure-induced transition is found to be a second-order transition. The band structure and density of states (DOS) are plotted for FM and NM states. Apart from this the ground-state properties like magnetic moment, lattice parameter and bulk modulus are calculated and are compared with the available results. Under large volume expansion these compounds exist in zinc-blende (ZB) structure, which shows half metallicity. The magnetic moment and equilibrium lattice constants for ZB structure are obtained as well as band structure and DOS are presented.
New valence electron photoelectron spectra of iodobenzene obtained using synchrotron radiation have been recorded. Ionization energies (IEs) determined using multi-configuration SCF calculation (MCSCF) procedures confirmed the adiabatic IE order as: X2B1
Palmer, Michael H., E-mail: m.h.palmer@ed.ac.uk, E-mail: tr01@staffmail.ed.ac.uk, E-mail: vronning@phys.au.dk, E-mail: nykj@phys.au.dk, E-mail: marcello.coreno@elettra.eu, E-mail: desimone@iom.cnr.it, E-mail: malgorzata.biczysko@sns.it; Ridley, Trevor [School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, Scotland (United Kingdom); Hoffmann, Søren Vrønning; Jones, Nykola C. [ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C (Denmark); Coreno, Marcello [CNR-IMIP, Montelibretti, c/o Laboratorio Elettra, Trieste (Italy); Simone, Monica de [CNR-IOM Laboratorio TASC, Trieste (Italy); Grazioli, Cesare [CNR-IOM Laboratorio TASC, Trieste (Italy); Department of Chemical and Pharmaceutical Sciences, University of Trieste, Trieste (Italy); Biczysko, Malgorzata [National Research Council ICCOM-CNR, UOS di Pisa, Via G. Moruzzi 1, I-56124 Pisa (Italy); Scuola Normale Superiore, Piazza Cavalieri 7, 56126 Pisa (Italy); Baiardi, Alberto [Scuola Normale Superiore, Piazza Cavalieri 7, 56126 Pisa (Italy)
2015-04-07
New valence electron photoelectron spectra of iodobenzene obtained using synchrotron radiation have been recorded. Ionization energies (IEs) determined using multi-configuration SCF calculation (MCSCF) procedures confirmed the adiabatic IE order as: X{sup 2}B{sub 1}studies. The calculated Franck-Condon vibrational spectral envelopes, including hot band contributions, for the first four ionic states reproduce the observed peak positions and intensities with reasonable accuracy. In order to simulate the observed spectra, different bandwidths are required for different states. The increase in the required bandwidths for the A{sup 2}A{sub 2} and B{sup 2}B{sub 2} states is attributed to internal conversion to lower-lying states. The presence of relatively high intensity sequence bands leads to asymmetry of each of the X{sup 2}B{sub 1} state bands.
Conformers and non-covalent interactions studied by laser spectroscopies and ab initio calculations
Ullrich, S
2001-01-01
The model peptides, formanilide and acetanilide, and their weakly bound complexes were studied in the gas-phase using resonance enhanced multi-photon ionisation (REMPI) and zero electron kinetic energy (ZEKE) photoelectron spectroscopy. Both, cis- and trans-isomers of formanilide, were observed under molecular beam conditions. Trans-formanilide displayed predominantly in-plane vibrational excitation indicative of a planar geometry with modest geometry changes upon excitation and ionisation. In cis-formanilide the side-chain is twisted compared to the phenyl plane in the S sub 0 state, but planar in the S sub 1 and D sub 0 states, revealed in characteristic side-chain torsional and out-of-plane bending excitations. Additionally, the ZEKE spectra provide evidence that excess cationic charge is delocalised from the aromatic ring to the side chain. The work on trans-formanilide was extended to its van der Waals complex with Argon with the purpose of investigating the torsional potential of the side-chain and prob...
The decoupling of epitaxial graphene on SiC by hydrogen intercalation: an ab initio study
Nemec, Lydia; Rinke, Patrick; Blum, Volker; Scheffler, Matthias
2015-03-01
Large-scale ordered epitaxial graphene can be grown on various substrates, out of which silicon carbide (SiC) is one of the most promising. The exact material properties of graphene depend on the growth conditions and its interaction with the substrate. By hydrogen intercalation of epitaxial graphene on the Si-face of SiC the graphene layer decouples from the substrate forming quasi-free-standing monolayer graphene (QFMLG). We performed an density functional theory study of QFMLG on the polar 6H-SiC(0001) surface based on a van der Waals corrected semi-local exchange-correlation functional using the all-electron numeric atom-centered basis function code FHI-aims. We find an adsorption height in excellent agreement with X-ray standing wave experiments, a very low buckling of the graphene layer, and a very homogeneous electron density at the interface. All these features improve the electronic properties of QFMLG compared to epitaxial graphene. Using the insight gleaned on the Si-face, we present the structure of a hypothetical QFMLG phase on the C-face of SiC. We find that hydrogen intercalation is a promising option to control the SiC-graphene interface.
Renner-Teller nonadiabatic coupling terms: An ab-initio study of the HNH molecule
Halász, G. J.; Vibók, Á.; Baer, R.; Baer, M.
2006-02-01
In this Communication we present the first theoretical/numerical treatment of nonadiabatic coupling terms (NACT) that originate from the Renner-Teller (RT) model, namely, those that follow from the splitting of an electronic level of a linear molecule when it becomes bent. These two newly formed states are characterized by different symmetries and are designated as A and B. Our main findings: (1) The RT NACTs are quantized as long as they are calculated close enough to collinear configuration of the molecule (in this case HNH). Their value is τ =1 (the Jahn-Teller values in similar situations, are τ =1/2). (2) Calculation of RT NACTs at bent configurations (i.e., at a distance from the linear axis) yield decreased values, sometimes by more than 50%. This last finding implies that in strongly bent configurations the two-state Hilbert subspace (formed by the above mentioned A and B states) is affected by upper states, most likely via Jahn-Teller conical intersections. (3) This study has also important practical implications. The fact that the RT NACTs decrease in (strongly) bent situations implies that analyzing spectroscopic data employing only the two Π-states may not be sufficient in order to achieve the required accuracy.
Effects of additives in {alpha}- and {theta}-alumina: an ab initio study
Wallin, Erik; Andersson, Jon M; Chirita, Valeriu; Helmersson, Ulf [Department of Physics, Linkoeping University, SE-581 83 Linkoeping (Sweden)
2004-12-15
It is of high fundamental and practical importance to be able to control the formation and stability of the different crystalline phases of alumina (Al{sub 2}O{sub 3}). In this study, we have used density functional theory methods to investigate the changes induced in the thermodynamically stable {alpha} phase and the metastable {theta} phase as one eighth of the Al atoms are substituted for different additives (Sc, W, Mo, Cr, Cu, Si, and B). The calculations predict that the additives strongly affect the relative stability between the two phases. Most tested additives are shown to shift the relative stability towards, and in some cases completely stabilize, the {theta} phase, while Cu doping is predicted to increase the relative stability of the {alpha} phase. The reasons for these effects are discussed, as are possible implications on the growth and use of doped aluminas in practical applications. In addition, the effects of the additives on bulk moduli and densities of states have been investigated.
Ab-initio study of C and O impurities in uranium nitride
Lopes, Denise Adorno; Claisse, Antoine; Olsson, Pär
2016-09-01
Uranium nitride (UN) has been considered a potential fuel for Generation IV (GEN-IV) nuclear reactors as well as a possible new fuel for Light Water Reactors (LWR), which would permit an extension of the fuel residence time in the reactor. Carbon and oxygen impurities play a key role in the UN microstructure, influencing important parameters such as creep, swelling, gas release under irradiation, compatibility with structural steel and coolants, and thermal stability. In this work, a systematic study of the electronic structure of UN containing C and O impurities using first-principles calculations by the Density Functional Theory (DFT) method is presented. In order to describe accurately the localized U 5f electrons, the DFT + U formalism was adopted. Moreover, to avoid convergence toward metastable states, the Occupation Matrix Control (OMC) methodology was applied. The incorporation of C and O in the N-vacancy is found to be energetically favorable. In addition, only for O, the incorporation in the interstitial position is energetically possible, showing some degree of solubility for this element in this site. The binding energies show that the pairs (Csbnd Nvac) and (Osbnd Nvac) interact much further than the other defects, which indicate the possible occurrence of vacancy drag phenomena and clustering of these impurities in grain boundaries, dislocations and free surfaces. The migration energy of an impurity by single N-vacancy show that C and O employ different paths during diffusion. Oxygen migration requires significantly lower energy than carbon. This fact is due to flexibility in the Usbnd O chemical bonds, which bend during the diffusion forming a pseudo UO2 coordination. On the other hand, C and N have a directional and inflexible chemical bond with uranium; always requiring the octahedral coordination. These findings provide detailed insight into how these impurities behave in the UN matrix, and can be of great interest for assisting the development of
Ab initio study of oxygen adsorption and initial incorporation in Pd-Pt alloy
Dianat, Arezoo; Bobeth, Manfred; Pompe, Wolfgang [Institut fuer Werkstoffwissenschaft, Technische Universitaet Dresden (Germany)
2008-07-01
In order to get insight into the catalytic behavior of the bimetallic alloy Pd-Pt, the adsorption of oxygen on the alloy surface as well as the initial oxygen incorporation have been studied by means of density-functional theory. Due to segregation processes the composition of the near-surface layers of the alloy is in general different from the bulk composition. In this work, ordered bulk phases (L1{sub 0},L1{sub 2}) of Pd{sub x}Pt{sub 1-x} with compositions x=0.25,0.5,0.75 as well as different compositions and atom configurations of the two outermost layers have been investigated. Adsorption energies for oxygen adsorbed on the (111) alloy surface and for oxygen in sub-surface position have been calculated for different oxygen coverages (0.25 to 1 ML). In the case of oxygen adsorption on the surfaces, the highest adsorption energy at low oxygen coverage ({<=}0.25 ML) has been found for the Pd{sub 0.5}Pt{sub 0.5} composition o f the two outermost layers, whereas at higher coverage the oxygen binding energy is highest for a Pd-rich first monolayer and Pt-rich second monolayer. Interestingly, on the latter layer system the oxygen adsorption energy is higher than on pure Pd(111). Oxygen occupation of sub-surface sites starts at a coverage of 0.5 ML for all alloy models considered. The calculated oxygen adsorption energy depends only weakly on the bulk composition. It increases slightly with Pt bulk concentration, but it differs less than 50 meV.
High-pressure polymorphs of TbVO{sub 4}: A Raman and ab initio study
Errandonea, D., E-mail: daniel.errandonea@uv.es [Departamento de Física Aplicada-ICMUV, Universidad de Valencia, MALTA Consolider Team, Edificio de Investigación, C/Dr. Moliner 50, Burjassot, 46100 Valencia (Spain); Manjón, F.J. [Instituto de Diseño para la Fabricación y Producción Automatizada, MALTA Consolider Team, Universidad Politécnica de Valencia, Camino de Vera s/n, 46022 Valencia (Spain); Muñoz, A.; Rodríguez-Hernández, P. [Departamento de Física Fundamental II, Instituto de Materiales y Nanotecnología, MALTA Consolider Team, Universidad de La Laguna, La Laguna 38205, Tenerife (Spain); Panchal, V. [Departamento de Física Aplicada-ICMUV, Universidad de Valencia, MALTA Consolider Team, Edificio de Investigación, C/Dr. Moliner 50, Burjassot, 46100 Valencia (Spain); Royal College of Arts, Science and Commerce, Mira Road, Mumbai 401 107 (India); Achary, S.N.; Tyagi, A.K. [Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085 (India)
2013-11-15
Highlights: •Three phase transitions are induced in zircon-type TbVO4 at 6.7, 26.7, and 34.4 GPa. •The proposed structural sequence is zircon-scheelite-fergusonite-orthorhombic Cmca. •Scheelite phase is metaestable after decompression. •The equation of states for all phases is reported. •Compressibility is enhanced in the Cmca phase due to f-electron delocalization. -- Abstract: Raman measurements on TbVO{sub 4} show the occurrence of three pressure-induced phase transitions. The first one, an irreversible transition from the zircon to the scheelite structure, occurs beyond 6.7 GPa. In addition, two reversible transformations take place at 26.7 and 34.4 GPa. The last transition was never reported before. The experimental findings are supported by structural and lattice-dynamics calculations that helped us to identify the post-scheelite phase as a monoclinic fergusonite structure. According to the calculations, the third transition involves a symmetry increase. An orthorhombic structure is proposed for the phase found above 34.4 GPa. The results have been compared with previous studies in TbVO{sub 4} and discussed in comparison with related compounds. The calculated equations of state are reported for the different polymorphs of TbVO{sub 4}. A compressibility increase is caused by the third transition. It is associated to a bond-strength decrease, which is related to a coordination increase and a delocalization of Tb f-electrons.
Origin of using cisplatin over transplatin for cancer treatment: An ab initio study
Li, Sa; Jena, Puru; Department of Physics, Virginia Commonwealth University Team
2011-03-01
Eventhough cisplatin has been used as a chemotherapy anti-cancer drug for over 40 years the thermodynamics and kinetics of the reactions are still largely unknown. Cisplatin molecules are known to be attacked by water molecules before they react with DNA. As a result, two Cl atoms are eliminated. The active piece in the cell, therefore, is not cisplatin but (NH3)2 Pt 2+ . To explain why only cisplatin but not transplatin functions as anticancer drug, we used first principles method to study the dechlorination process in cis- and transplatin. Although transplatin molecule is more stable than cisplatin by 0.52 eV, we found cisplatin to be more favorable for reaction due to the following reasons: 1) the energy cost to remove a Cl atom is less from cisplatin than transplatin. 2) cis-form (NH3)2 Pt 2+ derived from cisplatin with N-Pt-N angle of 97r is lower in energy than trans-form derived from transplatin with N-Pt-N angle of 180r. The rotation barrier for N-Pt-N changing from 180r to 97r is about 1.0 eV. 3) When cis-form of (NH3)2 Pt 2+ reacts with two Guanines in DNA, the two N atoms in Guanines can readily bind to the Pt atom in cisplatin. The transplatin due to steric reasons does not provide that opportunity. This work is supported by grants from the Department of Energy.
Ab initio study of gas phase and water-assisted tautomerization of maleimide and formamide
Susheel Kalia; Anju Sharma; B S Kaith
2007-11-01
Maleimide serves as an important starting material in the synthesis of drugs and enzyme inhibitors. In the present paper, knowing the importance of tautomerization in maleimide for its drug action, potential energy surface of maleimide is studied and its tautomerization has been discussed and compared with tautomerization of formamide. Gas phase tautomerization of maleimide requires large amount of energy (23.21 kcal/mol) in comparison to formamide (15.05 kcal/mol) at HF/6-31+G* level. Thus making the proton transfer reaction a difficult process in gas phase. Water molecule lowers the energy barrier of tautomerization thus facilitating the tautomerization of maleimide to 5-hydroxy-pyrrol-2-one. Water assisted tautomerization of maleimide requires 19.60 kcal/mol energy at HF/6-31+G* and 17.63 kcal/mol energy at B3LYP/6-31+G* level, a decrease of 3.61 and 5.96 kcal/mol over gas phase tautomerization. Whereas, tautomerization of formamide requires 14.16 and 12.84 kcal/mol energy, a decrease of 0.89 and 2.01 kcal/mol energy over gas phase tautomerization at HF/6-31+G* and B3LYP/6-31+G* level, respectively. Water-assisted tautomerization in maleimide and formamide showed that difference in energy barrier reduces to 2.83 kcal/mol from 10.41 kcal/mol (in gas phase) at B3LYP level, which resulted that maleimide readily undergoes tautomerization in water molecule.
Synthesis, bioassay, crystal structure and ab initio studies of Erlenmeyer azlactones
Parveen, Mehtab; Ali, Akhtar; Ahmed, Sarfaraz; Malla, Ali Mohammed; Alam, Mahboob; Pereira Silva, P. S.; Silva, Manuela Ramos; Lee, Dong-Ung
2013-03-01
Several 4-arylidene-2-phenyl-5(4H)-azlactones have been synthesized via Erlenmeyer method. The synthesized compounds have been characterized on the basis of systematic spectral studies (IR, 1H NMR, 13C NMR, and MS). The compound (4Z)-4-(3,5-dimethoxybenzylidene)-2-phenyl-1,3-oxazol-5(4H)-one, C18H15NO4, (5), crystallizes in the orthorhombic system, space group P212121, with a = 5.6793(3) Å, b = 15.2038(7) Å, c = 17.6919(10) Å, Mr = 309.31, V = 1527.64(14) Å3, Z = 4 and R = 0.0547. The compound (4Z)-2-phenyl-4-(3,4,5-trimethoxybenzylidene)-1,3-oxazol-5(4H)-one, C19H17NO5, (6) crystallizes in triclinic geometry with space group P-1, having unit cell parameters a = 7.3814(3) Å, b = 8.1446(3) Å, c = 13.9845(5) Å, α = 86.918(3), β = 83.314(2), γ = 82.462(3), Mr = 339.34, V = 827.16(5) Å3, Z = 2 and R = 0.0433. The DFT calculations of compounds (5) and (6) have been carried out to ascertain the stability of Z-conformer. The in vitro antimicrobial activity of all the compounds (1-6) was evaluated by the disk diffusion method against gram +ve and gram -ve microorganism and fungal strains. The MIC of the synthesized compounds was determined by agar well diffusion method in 96-well microtiter plate. All the synthesized compounds were also screened for their free radical scavenging activity by DPPH method.
无
2001-01-01
The static O-H bond parameters including O-H bond length, O-H charge difference, O-H Mulliken population and O-H bond stretching force constant (k) for 17 phenols were calculated by ab initio method HF/6-31G**. In combination with the O-H bond dissociation enthalpies (BDE) of the phenols determined by experiment, it was found that there were poor correlationships between the static O-H bond parameters and O-H BDE. Considering the good correlationship between O-H BDE and logarithm of free radical scavenging rate constant for phenolic antioxidant, it is reasonable to believe that the ineffectiveness of static O-H bond parameters in characterizing antioxidant activity arises from the fact that they cannot measure the O-H BDE.
Ab initio study of 59Co NMR spectra in Co2FeAl1-xSix Heusler alloys
Nishihara, H.; Sato, K.; Akai, H.; Takiguchi, C.; Geshi, M.; Kanomata, T.; Sakon, T.; Wada, T.
2015-05-01
Ab initio electronic structure calculation of a series of Co2FeAl1-xSix Heusler alloys has been performed, using the Korringa-Kohn-Rostoker-coherent potential approximation method to explain experimental 59Co NMR spectra. Two prominent features are explained semi-quantitatively-a global shift of the 59Co resonance line due to alloying with Al and Si atoms in Co2FeAl1-xSix, and the effect of local disorder in creating distinct satellite lines of 59Co NMR in Co2FeAl. The importance is stressed of the positive contribution to the 59Co hyperfine field from valence electron polarization, which emerges from the half-metallic band structure inherent in Co-based Heusler alloys.
The (Lb)S1 0 transition of phenylpropyne and phenylacetylene: an experimental and ab initio study
This work describes the first excited singlet electronic state, Lb, of phenylpropyne (PPR) and phenylacetylene (PA). Ab initio calculations have been performed for the geometry and normal modes in the S0 and S1 states. One-photon and two-photon S1 0 spectra of jet cooled samples have been examined (REMPI detection) and a detailed vibronic analysis has been carried out for PPR and PA. The origin band of PPR (λ = 279.64 nm) is one order of magnitude weaker than the origin band (λ = 278.63 nm) of PA and the acetylenic modes are greatly reduced by the H → CH3 substitution. These changes and the parallel PPR-PA comparison provided a way for secure vibronic assignment in both molecules, which was also aided by theoretical predictions
Sousa, J. B.; Calheiros, R.; Rio, V.; Borges, F.; Marques, M. P. M.
2006-02-01
A conformational analysis of ethyl 3-(3,4,5-trihydroxyphenyl)-2-propenoate (ethyl 3,4,5-trihydroxycinnamate, ETHPPE), a polyphenolic cinnamic ester which displays antiproliferative activity towards human adenocarcinoma cells, was carried out by Raman spectroscopy coupled to ab initio MO calculations. Apart from the optimised geometrical parameters for the most stable conformations of this compound (both for the trans and cis isomers), the corresponding harmonic vibrational frequencies were obtained. Eighteen distinct geometries were found, 12 for the lowest energy trans isomer and six for the cis species. The conformational preferences of this system were verified to be mainly ruled by the stabilising effect of π-electron delocalisation, a planar geometry being favoured. The orientation of the ester moiety showed to be the most determinant factor for the overall stability of the molecule. In the light of these results, a complete assignment of the corresponding Raman pattern was performed.
Koukaras, Emmanuel N; Zdetsis, Aristides D; Sigalas, Michael M
2012-09-26
On the basis of the attractive possibility of efficient hydrogen storage in light metal hydrides, we have examined a large variety of Mg(n)H(m) nanoclusters and (MgH(2))(n) nanocrystals (n = 2-216, m = 2-436) using high level coupled cluster, CCSD(T), ab initio methods, and judicially chosen density functional calculations of comparable quality and (near chemical) accuracy. Our calculated desorption energies as a function of size and percentage of hydrogen have pinpointed optimal regions of sizes and concentrations of hydrogen which are in full agreement with recent experimental findings. Furthermore, our results reproduce the experimental desorption energy of 75.5 kJ/mol for the infinite system with remarkable accuracy (76.5 ± 1.5 kJ/mol). PMID:22920498
An ab initio study of the size-dependent mechanical behavior of single-walled AlN nanotubes
Hao, Jun-Hua; Wang, Yu-Fang; Yin, Yu-Hua; Jiang, Run; Wang, Yun-Feng; Jin, Qing-Hua
2015-07-01
Employing ab initio electronic structure calculations combined with the linear combination of atomic orbitals (LCAO) we have investigated a size dependence of mechanical behavior in single-walled AlN nanotubes with armchair and zigzag forms. A simple procedure of nanotubes construction based on the wurtzite (0 0 1) slab with monolayer rolling and subsequent cylindrical coordinate system introduction is suggested. The present calculations indicate that the Young's modulus and electronic band gap of these tubes are increased monotonically as the radius increases, but decreases with the Al-N bond length. In addition, the amount of charge transfer calculated by the Mulliken's population analysis is introduced to explain clearly the strength of bonding between Al and N atoms in single-walled AlN nanotubes.
Ab-initio calculation study on the formation mechanism of boron-oxygen complexes in c-Si
Boron-oxygen (B-O) complex in crystalline silicon (c-Si) solar cells is responsible for the light-induced efficiency degradation of solar cell. However, the formation mechanism of B-O complex is not clear yet. By Ab-initio calculation, it is found that the stagger-type oxygen dimer (O2ist) should be the component of B-O complex, whose movement occurs through its structure reconfiguration at low temperature, instead of its long-distance diffusion. The O2ist can form two stable “latent centers” with the Bs, which are recombination-inactive. The latent centers can be evolved into the metastable recombination centers via their structure transformation in the presence of excess carriers. These results can well explain the formation behaviors of B-O complexes in c-Si
Song, Chi [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031 (China); Li, Dongdong [National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei 230026 (China); Xu, Yichun [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031 (China); Pan, B.C. [National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei 230026 (China); Liu, C.S., E-mail: csliu@issp.ac.cn [Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129, Hefei 230031 (China); Wang, Zhiguang [Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000 (China)
2013-11-15
Considerable attention has been devoted to liquid lead-bismuth eutectic (LBE) alloy due to its potential application as spallation target and coolant in future sub-critical reactors. Whether there exists an abnormal structural change at high temperatures in this liquid alloy is still under debate. In this paper, we perform ab initio molecular dynamics simulation on the structure and dynamics of liquid LBE at different temperatures from 573 to 1173 K. Through the analysis of the pair correlation function, static structure factor, coordination number, atomic bonded pair, excess entropy, and diffusion constant with increasing temperature, we find that these structure-sensitive quantities change gradually with temperature and exhibit linear temperature dependence. No abnormal structural transformations with temperature variation are observed.
Devi, Assa Aravindh Sasikala
2014-05-01
Investigations on freestanding binary and ternary clusters of Fe (x) Co (y) Ir (z) (x + y + z = 5, 6) are carried out using ab initio density functional theory techniques. The geometry, chemical order, binding energy, magnetic moment and electronic structure of the clusters are analyzed for the entire range of composition. Composition dependent structural transition is observed in the five atom clusters, while octahedral geometry prevailed in clusters with six atoms. Both the clusters show increment in binding energy with the increase in number of heterogeneous bonds. Analysis based on the chemical order parameter indicates that clusters favor mixing rather than segregation. The clusters exhibit ferromagnetic ordering and the inter-dependence of optimal cluster geometry to the magnetic moments and electronic structure is observed.
Ab initio gene identification in metagenomic sequences.
Zhu, Wenhan; Lomsadze, Alexandre; Borodovsky, Mark
2010-07-01
We describe an algorithm for gene identification in DNA sequences derived from shotgun sequencing of microbial communities. Accurate ab initio gene prediction in a short nucleotide sequence of anonymous origin is hampered by uncertainty in model parameters. While several machine learning approaches could be proposed to bypass this difficulty, one effective method is to estimate parameters from dependencies, formed in evolution, between frequencies of oligonucleotides in protein-coding regions and genome nucleotide composition. Original version of the method was proposed in 1999 and has been used since for (i) reconstructing codon frequency vector needed for gene finding in viral genomes and (ii) initializing parameters of self-training gene finding algorithms. With advent of new prokaryotic genomes en masse it became possible to enhance the original approach by using direct polynomial and logistic approximations of oligonucleotide frequencies, as well as by separating models for bacteria and archaea. These advances have increased the accuracy of model reconstruction and, subsequently, gene prediction. We describe the refined method and assess its accuracy on known prokaryotic genomes split into short sequences. Also, we show that as a result of application of the new method, several thousands of new genes could be added to existing annotations of several human and mouse gut metagenomes. PMID:20403810
Ab initio two-component Ehrenfest dynamics
We present an ab initio two-component Ehrenfest-based mixed quantum/classical molecular dynamics method to describe the effect of nuclear motion on the electron spin dynamics (and vice versa) in molecular systems. The two-component time-dependent non-collinear density functional theory is used for the propagation of spin-polarized electrons while the nuclei are treated classically. We use a three-time-step algorithm for the numerical integration of the coupled equations of motion, namely, the velocity Verlet for nuclear motion, the nuclear-position-dependent midpoint Fock update, and the modified midpoint and unitary transformation method for electronic propagation. As a test case, the method is applied to the dissociation of H2 and O2. In contrast to conventional Ehrenfest dynamics, this two-component approach provides a first principles description of the dynamics of non-collinear (e.g., spin-frustrated) magnetic materials, as well as the proper description of spin-state crossover, spin-rotation, and spin-flip dynamics by relaxing the constraint on spin configuration. This method also holds potential for applications to spin transport in molecular or even nanoscale magnetic devices
Toward ab initio density functional theory for nuclei
Drut, J. E.; Furnstahl, R. J.; Platter, L.
2009-01-01
We survey approaches to nonrelativistic density functional theory (DFT) for nuclei using progress toward ab initio DFT for Coulomb systems as a guide. Ab initio DFT starts with a microscopic Hamiltonian and is naturally formulated using orbital-based functionals, which generalize the conventional local-density-plus-gradients form. The orbitals satisfy single-particle equations with multiplicative (local) potentials. The DFT functionals can be developed starting from internucleon forces using ...
Spinel indium sulphide exists in three phases. The tetragonal β-phase transforms to the cubic α-phase at 420 °C which further transforms to the trigonal γ-phase at 754 °C. Due to wide energy bandgap, the phases of indium sulphide have possibilities of applications in photo-electrochemical solar cell devices as a replacement of toxic CdS. The electronic, optical and transport properties of the three phases have therefore been investigated using full potential linear augmented plane wave (FP-LAPW) + local orbitals (lo) scheme, in the framework of density functional theory (DFT) with generalized gradient approximation (GGA) for the purpose of exchange-correlation energy functional. We present the structure, energy bands and density of states (DOS) for α-, β- and γ-phases. The partial density of states (PDOS) of β-In2S3 is in good agreement with experiment and earlier ab initio calculations. To obtain the fundamental characteristics of these phases we have analysed their linear optical properties such as the dynamic dielectric function in the energy range of 0–15 eV. From the dynamic dielectric function it is seen that there is no directional anisotropy for α-phase since the longitudinal and transverse components are almost identical, however the β and γ-phases show birefringence. The optical absorption profiles clearly indicate that β-phase has possibility of greater multiple direct and indirect interband transitions in the visible regions compared to the other phases. To study the existence of interesting thermoelectric properties, transport properties like electrical and thermal conductivities, Seebeck and Hall coefficients etc. are also calculated. Good agreements are found with the available experimental results. -- Highlights: ► The electronic properties of phases of In2S3 have been investigated. ► The phases exhibit luminescence properties due to vacancies in crystal structure. ► The phases of In2S3 have low thermal conductivity and high
Sharma, Yamini, E-mail: sharma.yamini62@gmail.com [Department of Physics, Feroze Gandhi College, Rae Bareli 229001, U.P. (India); Srivastava, Pankaj [Department of Physics, Feroze Gandhi College, Rae Bareli 229001, U.P. (India)
2012-08-15
Spinel indium sulphide exists in three phases. The tetragonal {beta}-phase transforms to the cubic {alpha}-phase at 420 Degree-Sign C which further transforms to the trigonal {gamma}-phase at 754 Degree-Sign C. Due to wide energy bandgap, the phases of indium sulphide have possibilities of applications in photo-electrochemical solar cell devices as a replacement of toxic CdS. The electronic, optical and transport properties of the three phases have therefore been investigated using full potential linear augmented plane wave (FP-LAPW) + local orbitals (lo) scheme, in the framework of density functional theory (DFT) with generalized gradient approximation (GGA) for the purpose of exchange-correlation energy functional. We present the structure, energy bands and density of states (DOS) for {alpha}-, {beta}- and {gamma}-phases. The partial density of states (PDOS) of {beta}-In{sub 2}S{sub 3} is in good agreement with experiment and earlier ab initio calculations. To obtain the fundamental characteristics of these phases we have analysed their linear optical properties such as the dynamic dielectric function in the energy range of 0-15 eV. From the dynamic dielectric function it is seen that there is no directional anisotropy for {alpha}-phase since the longitudinal and transverse components are almost identical, however the {beta} and {gamma}-phases show birefringence. The optical absorption profiles clearly indicate that {beta}-phase has possibility of greater multiple direct and indirect interband transitions in the visible regions compared to the other phases. To study the existence of interesting thermoelectric properties, transport properties like electrical and thermal conductivities, Seebeck and Hall coefficients etc. are also calculated. Good agreements are found with the available experimental results. -- Highlights: Black-Right-Pointing-Pointer The electronic properties of phases of In{sub 2}S{sub 3} have been investigated. Black-Right-Pointing-Pointer The
Ab initio study on the adsorption mechanism of oxygen on Cr2AlC (0 0 0 1) surface
Highlights: • Density-functional calculations have been performed on Cr2AlC (0 0 0 1) surface and in contact with oxygen adsorption, which is a very important step towards understanding the oxidation processes on the MAX phases. • For the study of the single O atoms adsorption, we observed the site preferences that partly resemble that of (1 1 1) of FCC−Al. • The energy barriers (NEB) for various diffusion paths to form a pair of Al-O clusters are identified and calculated showing the favorable formation of short-range alumina clusters as a prelude for a continuous layer formation. • In both cases of the adsorption systems (O and O2), the general picture are the quite similar, namely, the adatom O strongly bonds to the surface Al and formed Al-O bond resulting in the redistribution of the Mulliken charges and the charge transfer from the Cr2AlC (0 0 0 1) surface to the adsorbed O atom. - Abstract: We performed an investigation of the early stage oxidation of the technologically relevant Cr2AlC (0 0 0 1) surface by ab initio calculations. In this work, the geometrical structure, bonding character, and electronic structure of the Cr2AlC (0 0 0 1) bare surface with or without single oxygen atom and molecular adsorption had been systematically investigated by DFT calculation. Four possible terminations Al-, Cr(C)-, C- and Cr(Al)-terminated Cr2AlC (0 0 0 1) surface are considered. The corresponding surface energies of the four configurations are compared, indicating that the Al- and Cr(C)-terminated surfaces are more stable than the other two cases C- and Cr(Al)-terminated surfaces. For the most stable case, Al-terminated Cr2AlC (0 0 0 1) surface, a detailed model describing the oxygen–surface interactions is developed by assessing the adsorption energetics of various adsorption mechanisms. Based on the evaluation of the energetics and the structural properties of the atomistic models generated, the results point to a consistent picture of the initial stage
Clerc, Daryl G
2016-07-21
An ab initio approach was used to study the molecular-level interactions that connect gene-mutation to changes in an organism׳s phenotype. The study provides new insights into the evolutionary process and presents a simplification whereby changes in phenotypic properties may be studied in terms of the binding affinities of the chemical interactions affected by mutation, rather than by correlation to the genes. The study also reports the role that nonlinear effects play in the progression of organs, and how those effects relate to the classical theory of evolution. Results indicate that the classical theory of evolution occurs as a special case within the ab initio model - a case having two attributes. The first attribute: proteins and promoter regions are not shared among organs. The second attribute: continuous limiting behavior exists in the physical properties of organs as well as in the binding affinity of the associated chemical interactions, with respect to displacements in the chemical properties of proteins and promoter regions induced by mutation. Outside of the special case, second-order coupling contributions are significant and nonlinear effects play an important role, a result corroborated by analyses of published activity levels in binding and transactivation assays. Further, gradations in the state of perfection of an organ may be small or large depending on the type of mutation, and not necessarily closely-separated as maintained by the classical theory. Results also indicate that organs progress with varying degrees of interdependence, the likelihood of successful mutation decreases with increasing complexity of the affected chemical system, and differences between the ab initio model and the classical theory increase with increasing complexity of the organism. PMID:27029513
Realistic modelling of water/solid interfaces from ab Initio molecular dynamics
Tocci, G.
2014-01-01
Water/solid interfaces are of utmost importance to a number of technological processes. Theoretical studies, based on ab initio approaches are suitable to unveil processes occurring at water/solid interfaces and can therefore be instrumental to delineate guidelines to improve the efficiency of these processes. In this thesis we study several systems of current interest using ab initio methods based on density functional theory (DFT). By going often beyond the use of standard DFT methods and a...
J. Rychlewski
1997-07-01
Full Text Available The conformation of dimethyl (R,R-tartrate has been analyzed on the basis of the single crystal X-ray diffraction method as well as by ab-initio quantum chemical studies. The results showed that the extended T conformation containing two planar hydroxyester moieties predominates in both ab-initio and X-ray studies. The lowest energy conformer in ab-initio calculations has C2 symmetry and hydrogen bonds between a hydroxyl group and the nearest carbonyl oxygen. The second in energetical sequence, with an energy difference of only 1.2 kcal/mol, is the asymmetrical conformer, which differs from the lowest energy form by the rotation of one of the ester groups by 180Ã‚Â°. Intramolecular OH...O hydrogen bonds observed in this rotamer again involve only proximal functional groups. This conformer is present in the crystal structure of the studied compound, although its conformation in the solid state is no longer stabilized by intramolecular hydrogen bonds of the type mentioned above. In the crystal, hydroxyl groups are mostly involved in intermolecular hydrogen bonds and form only a weak intramolecular hydrogen bond with each other. The planar arrangement of the ÃŽÂ±-hydroxyester moieties combined with the extended conformation of the carbon chain seems to be stabilized by the intramolecular hydrogen bonds between neighboring functional groups and by the long range dipole-dipole interactions between two pairs of CO and (ÃŽÂ²C-H bonds.
Highlights: ► A theoretical study of hydroxide ion-water clusters is carried for varying cluster size and temperature. ► The structures of OH−(H2O)n are found out through quantum chemical calculations for n = 4, 8, 16 and 20. ► The finite temperature behavior of the clusters is studied through ab initio dynamical simulations. ► The spectral features of OH modes (deuterated) and their dependence on hydrogen bonding states of water are discussed. ► The mechanism and kinetics of proton transfer processes in these anionic clusters are also investigated. - Abstract: We have investigated the hydration structure and dynamics of OH−(H2O)n clusters (n = 4, 8, 16 and 20) by means of quantum chemical and ab initio molecular dynamics calculations. Quantum chemical calculations reveal that the solvation structure of the hydroxide ion transforms from three and four-coordinated surface states to five-coordinated interior state with increase in cluster size. Several other isomeric structures with energies not very different from the most stable isomer are also found. Ab initio simulations show that the most probable configurations at higher temperatures need not be the lowest energy isomeric structure. The rates of proton transfer in these clusters are found to be slower than that in bulk water. The vibrational spectral calculations reveal distinct features for free OH (deuterated) stretch modes of water in different hydrogen bonding states. Effects of temperature on the structural and dynamical properties are also investigated for the largest cluster considered here.
Ab-initio study of the electronic structure of sup 1 sup 9 F implanted in GaAs and GaN crystals
Park, J H; Cho, H S; Shin, Y N
1998-01-01
We have studied the nuclear quadrupole interaction of a fluorine atom implanted in gallium arsenide and gallium nitride cluster models using the ab-initio Hartree-Fock theory. For the three possible fluorine sites in GaAs and GaN, we have determined the location of the implanted fluorine atom by using a self-consistent calculation, the electric field gradient at the implanted atom, and the electronic structure. Good agreement is found with experimental data wherever they are available. Predictions are made for the implanted fluorine site associated with the total energy and the electric field gradient which are expected to be measurable by a variety of experimental techniques.
Illas i Riera, Francesc; Rubio Martínez, Jaime; Ricart, J M
1985-01-01
The interaction of atomic F and Cl with Si4H9 and Ge4H9 cluster models has been studied by using ab initio pseudopotentials and basis sets of increasing complexity. The results show that the effect of d orbitals is important in order to reproduce the experimental findings. However, the use of polarization functions in the atoms which are directly involved in the chemisorption bond leads to results which are very close to those obtained using extended basis sets. The local nature of the chemis...
The elastic properties of fcc Fe–Mn–X (X = Cr, Co, Ni, Cu) alloys with additions of up to 8 at.% X were studied by combinatorial thin film growth and characterization and by ab initio calculations using the disordered local moments (DLM) approach. The lattice parameter and Young’s modulus values change only marginally with X. The calculations and experiments are in good agreement. We demonstrate that the elastic properties of transition metal alloyed Fe–Mn can be predicted by the DLM model. (paper)
Raybaud, P.; Hafner, J.; Kresse, G.; Toulhoat, H.
1998-02-01
The adsorption of thiophene on the catalytically active MoS2(010) surfaces has been studied using ab initio local-density-functional molecular dynamics. It is shown that thiophene adsorbs in an η5 configuration with the molecular ring parallel to the surface, centered above a coordinatively unsaturated Mo atom, and with the sulfur atom in a binding position between two Mo surface atoms. This configuration provides not only the highest adsorption energy, but activates in addition the thiophene molecule with respect to both C-S bond cleavage (and hence desulfurization) and hydrogenation. Hence it represents a realistic scenario for the first step in catalytic hydrodesulfurization.
Ab initio study of the adsorption, diffusion, and intercalation of alkali metal adatoms on the (0001) step surface of the topological insulator Bi2Se3 has been performed for the case of low coverage. The calculations of the activation energies of diffusion of adatoms on the surface and in van der Waals gaps near steps, as well as the estimate of diffusion lengths, have shown that efficient intercalation through steps is possible only for Li and Na. Data obtained for K, Rb, and Cs atoms indicate that their thermal desorption at high temperatures can occur before intercalation. The results have been discussed in the context of existing experimental data
Zemen, J., E-mail: zemen@fzu.cz [School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD (United Kingdom); Institute of Physics ASCR, v. v. i., Cukrovarnická 10, 162 00 Praha 6 (Czech Republic); Mašek, J. [Institute of Physics ASCR, v. v. i., Na Slovance 2, 182 21 Praha 8 (Czech Republic); Kučera, J. [Institute of Physics ASCR, v. v. i., Cukrovarnická 10, 162 00 Praha 6 (Czech Republic); Mol, J.A. [Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft (Netherlands); Institute of Physics ASCR, v. v. i., Cukrovarnická 10, 162 00 Praha 6 (Czech Republic); Motloch, P. [Institute of Physics ASCR, v. v. i., Cukrovarnická 10, 162 00 Praha 6 (Czech Republic); Jungwirth, T. [Institute of Physics ASCR, v. v. i., Cukrovarnická 10, 162 00 Praha 6 (Czech Republic); School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD (United Kingdom)
2014-04-01
An empirical multiorbital (spd) tight binding (TB) model including magnetism and spin–orbit coupling is applied to calculations of magnetic anisotropy energy (MAE) in CoPt L1{sub 0} structure. A realistic Slater–Koster parametrisation for single-element transition metals is adapted for the ordered binary alloy. Spin magnetic moment and density of states are calculated using a full-potential linearised augmented plane-wave (LAPW) ab initio method and our TB code with different variants of the interatomic parameters. Detailed mutual comparison of this data allows for determination of a subset of the compound TB parameters tuning of which improves the agreement of the TB and LAPW results. MAE calculated as a function of band filling using the refined parameters is in broad agreement with ab initio data for all valence states and in quantitative agreement with ab initio and experimental data for the natural band filling. Our work provides a practical basis for further studies of relativistic magnetotransport anisotropies by means of local Green's function formalism which is directly compatible with our TB approach. - Highlights: • Calculations of electronic structure properties of bulk ordered CoPt alloy using tight-binding (TB) and density functional theory (DFT) approach. • Refinement of existing single-element TB parameters for a binary alloy based on a comparison of band structure and spin magnetic moment per atom to DFT results. • Quantitative agreement of magnetic anisotropy energy (MAE) obtained by TB and DFT on a range of band fillings. • Successful description of ground state spin–orbit coupling phenomena using an extended TB model suitable for subsequent magnetotransport simulations.
An empirical multiorbital (spd) tight binding (TB) model including magnetism and spin–orbit coupling is applied to calculations of magnetic anisotropy energy (MAE) in CoPt L10 structure. A realistic Slater–Koster parametrisation for single-element transition metals is adapted for the ordered binary alloy. Spin magnetic moment and density of states are calculated using a full-potential linearised augmented plane-wave (LAPW) ab initio method and our TB code with different variants of the interatomic parameters. Detailed mutual comparison of this data allows for determination of a subset of the compound TB parameters tuning of which improves the agreement of the TB and LAPW results. MAE calculated as a function of band filling using the refined parameters is in broad agreement with ab initio data for all valence states and in quantitative agreement with ab initio and experimental data for the natural band filling. Our work provides a practical basis for further studies of relativistic magnetotransport anisotropies by means of local Green's function formalism which is directly compatible with our TB approach. - Highlights: • Calculations of electronic structure properties of bulk ordered CoPt alloy using tight-binding (TB) and density functional theory (DFT) approach. • Refinement of existing single-element TB parameters for a binary alloy based on a comparison of band structure and spin magnetic moment per atom to DFT results. • Quantitative agreement of magnetic anisotropy energy (MAE) obtained by TB and DFT on a range of band fillings. • Successful description of ground state spin–orbit coupling phenomena using an extended TB model suitable for subsequent magnetotransport simulations
Song, Lingchun; Han, Jaebeom; Lin, Yen-lin; Xie, Wangshen; Gao, Jiali
2009-01-01
The explicit polarization (X-Pol) method has been examined using ab initio molecular orbital theory and density functional theory. The X-Pol potential was designed to provide a novel theoretical framework for developing next-generation force fields for biomolecular simulations. Importantly, the X-Pol potential is a general method, which can be employed with any level of electronic structure theory. The present study illustrates the implementation of the X-Pol method using ab initio Hartree—Fo...
Raybaud, P.
1998-10-28
Ab-initio calculations within the DFT and GGA have been carried out in an attempt to understand better which property sets the activity of transition metal sulfides (TMS) in the hydro-desulfurization reaction (HDS), a most important step in the refining of crude oil. A systematic study of the structural cohesive and electronic properties of more than thirty bulk TMS has allowed us to find a new simple relationship between the experimental catalytic activities known in HDS, and the properly defined sulfur-metal (S-M) bond energy. On this volcano curve reminiscent of the Sabatier principles, ionic metallic sulfides (as Ni{sub 3}S{sub 2}, Ci{sub 9}S{sub 8}) exhibit the weakest bonds whereas ion-covalent semi-conductors (as MoS{sub 2}) exhibit the strongest bonds: the highest activities correspond to intermediate bond strengths (RuS{sub 2}). Our study of the electronic structure of the MoS{sub 2} edge surfaces has revealed acceptor surface states localized on Coordinatively Unsaturated Mo ions and the significance of those states for the activation of hetero-aromatic molecules like thiophene. On such surfaces, the energetic profiles we establish for the thiophene HDS reaction point out the surface anionic vacancy regeneration steps as rate determining. We have calculated the optimal positions of Co (Ni) in decoration on the MoS{sub 2} edge planes, in excellent agreement with available EXAFS data on real catalysts. Introducing the promoter Co (Ni) induces a lower optimal sulfur coverage and a lower surface S-M bond strength in proportion of the Co (Ni) coverage, and lower for Ni than for Co. Simplified energy profiles for the thiophene HDS reactions on promoted (101-bar 0) surfaces show that the C-S scission step is likely to become rate determining. Our results show altogether that bulk and surface S-M strengths in TMS systems show similar trends, insofar as they are primarily determined by the local electronic structure. (author)
Niazazari, Naser; Zatikyan, Ashkhen L.; Markarian, Shiraz A.
2013-06-01
The hydrogen bonding of 1:1 complexes formed between L-ascorbic acid (LAA) and dimethylsulfoxide (DMSO) has been studied by means of ab initio and density functional theory (DFT) calculations. Solutions of L-ascorbic acid (AA) in dimethylsulfoxide (DMSO) have been studied by means of both FT-IR (4000-220 cm-1) and FT-Raman spectroscopy. Ab initio Hartree-Fock (HF) and DFT methods have been used to determine the structure and energies of stable conformers of various types of L-AA/DMSO complexes in gas phase and solution. The basis sets 6-31++G∗∗ and 6-311+G∗ were used to describe the structure, energy, charges and vibrational frequencies of interacting complexes in the gas phase. The optimized geometric parameters and interaction energies for various complexes at different theories have been estimated. Binding energies have been corrected for basis set superposition error (BSSE) and harmonic vibrational frequencies of the structures have been calculated to obtain the stable forms of the complexes. The self-consistent reaction field (SCRF) has been used to calculate the effect of DMSO as the solvent on the geometry, energy and charges of complexes. The solvent effect has been studied using the Onsager models. It is shown that the polarity of the solvent plays an important role on the structures and relative stabilities of different complexes. The results obtained show that there is a satisfactory correlation between experimental and theoretical predictions.
Density functional theory and ab initio methods are employed to investigate decomposition pathways of 1,3,3-trinitroazetidine initiated by unimolecular loss of NO2 or HONO. Geometry optimizations are performed using M06/cc-pVTZ and coupled-cluster (CC) theory with single, double, and perturbative triple excitations, CCSD(T), is used to calculate accurate single-point energies for those geometries. The CCSD(T)/cc-pVTZ energies for NO2 elimination by N–N and C–N bond fission are, including zero-point energy (ZPE) corrections, 43.21 kcal/mol and 50.46 kcal/mol, respectively. The decomposition initiated by trans-HONO elimination can occur by a concerted H-atom and nitramine NO2 group elimination or by a concerted H-atom and nitroalkyl NO2 group elimination via barriers (at the CCSD(T)/cc-pVTZ level with ZPE corrections) of 47.00 kcal/mol and 48.27 kcal/mol, respectively. Thus, at the CCSD(T)/cc-pVTZ level, the ordering of these four decomposition steps from energetically most favored to least favored is: NO2 elimination by N–N bond fission, HONO elimination involving the nitramine NO2 group, HONO elimination involving a nitroalkyl NO2 group, and finally NO2 elimination by C–N bond fission
Ono, Yoshio; Arai Mitsuru; Tamura [The University of Tokyo, Tokyo (Japan). Department of Chemical system Engineering; Matsunaga, Takehiro [National Institute of Materials and Chemical Research, Tsukuba (Japan)
1999-10-31
In order to estimate the thermal stability of tetrazoles, ab initio MO calculations of alkyl tetrazoles were carried out and the initial process of thermal decomposition were discussed. The relationships between the chemical structures and the thermal stabilities were investigated. The initial step of the thermal decomposition is the cleavage of the N3-N4 bond, then the C5-N1 bond is cleave. The relationship between the N3-N4 bond length and the thermal stability is recognized, that is, the shorter the bond length is, the more thermally stable the tetrazole is. In addition it turns out, the more positive the ring charge is, the more thermally stable the tetrazole is. The relationship between the {pi}-electron density and the thermal stability is not found. The indexes adopted as a measure of thermal stability indicate that the more equal the bond lengths of the ring are, the more thermally stable they are. From these results, three tetrazoles with both high thermal stability and high nitrogen content are proposed. (author)
Taming the resistive switching in Fe/MgO/V/Fe magnetic tunnel junctions: An ab initio study
Aguiar-Hualde, J.M. [IPhT, CEA/Saclay, Orme des Merisiers, 91190 Gif-sur-Yvette Cedex (France); Alouani, M. [IPCMS, UMR 7504 CNRS-UdS, 23 rue du Loess, Strasbourg 67034 (France)
2014-12-15
A possible mechanism for the resistive switching observed experimentally in Fe/MgO/V/Fe junctions is presented. Ab initio total energy calculations within the local density approximation and pseudopotential theory shows that by moving the oxygen ions across the MgO/V interface one obtains a metastable state. It is argued that this state can be reached by applying an electric field across the interface. In addition, the ground state and the metastable state show different electric conductances. The latter results are discussed in terms of the changes of the density of states at the Fermi level and the charge transfer at the interface due to the oxygen ion motion. - Highlights: • Local minima are found for oxygen near the interface with at least one oxygen moved. • Relaxation of a small unit cell preserves this result and lowers energy barrier. • V on the top of Mg exhibits the minimum and a reasonable energy barrier. • Sense of switching: experimental evidence of the configuration (V on O or V on Mg). • Sense of switching can be understood in terms of charge oscillations induced by the O.
Long, Run; Fang, Weihai; Akimov, Alexey V
2016-02-18
We report ab initio time-domain simulations of nonradiative electron-hole recombination and electronic dephasing in ideal and defect-containing monolayer black phosphorus (MBP). Our calculations predict that the presence of phosphorus divacancy in MBP (MBP-DV) substantially reduces the nonradiative recombination rate, with time scales on the order of 1.57 ns. The luminescence line width in ideal MBP of 150 meV is 2.5 times larger than MBP-DV at room temperature, and is in excellent agreement with experiment. We find that the electron-hole recombination in ideal MBP is driven by the 450 cm(-1) vibrational mode, whereas the recombination in the MBP-DV system is driven by a broad range of vibrational modes. The reduced electron-phonon coupling and increased bandgap in MBP-DV rationalize slower recombination in this material, suggesting that electron-phonon energy losses in MBP can be minimized by creating suitable defects in semiconductor device material. PMID:26821943
Vázquez-Mayagoitia, Alvaro; Huertas, Oscar; Brancolini, Giorgia; Migliore, Agostino; Sumpter, Bobby G; Orozco, Modesto; Luque, F Javier; Di Felice, Rosa; Fuentes-Cabrera, Miguel
2009-10-29
The structural, tautomeric, hydrogen-bonding, stacking, and electronic properties of a seleno-derivative of thymine (T), denoted here as 4SeT and created by replacing O4 in T with Se, are investigated by means of ab initio computational techniques. The structural properties of T and 4SeT are very similar, and the geometrical differences are mainly limited to the adjacent environment of the C-Se bond. The canonical "keto" form is the most stable tautomer, in the gas phase and in aqueous solution, for both T and 4SeT. It is argued that the competition between two opposite trends, i.e., a decrease in the base-pairing ability and an increase of the stacking interaction upon incorporation of 4SeT into a duplex, likely explains the similar experimental melting points of a seleno-derivative duplex (Se-DNA) and its native counterpart. Interestingly, the underlying electronic structure shows that replacement of O4 with Se promotes a reduction in the HOMO-LUMO gap and an increase in interplane coupling, which suggests that Se-DNA could be potentially useful for nanodevice applications. This finding is further supported by the fact that transfer integrals between 4SeT...A stacked base pairs are larger than those determined for similarly stacked natural T...A pairs. PMID:19813710
Vazquez-Mayagoitia, Alvaro [ORNL; Fuentes-Cabrera, Miguel A [ORNL; Sumpter, Bobby G [ORNL; Luque, Javier [Universitat de Barcelona; Huertas, Oscar [Universitat de Barcelona; Orozco, Modesto [Institut de Recerca Biomedica, Parc Cientific de Barcelona, Barcelona, Spain; Felice, Rosa [INFM-CNR National Research Center S3; Brancolini, Giorgia [ORNL; Migliore, Agostino [University of Pennsylvania
2009-01-01
The structural, tautomeric, hydrogen-bonding, stacking and electronic properties of a seleno-derivative of thymine (T), denoted here as 4SeT and created by replacing O4 in T with Se, are investigated by means of ab initio computational techniques. The structural properties of T and 4SeT are very similar and the geometrical differences are mainly limited to the adjacent environment of the C-Se bond. The canonical keto form is the most stable tautomer, in gas phase and in aqueous solution, for both T and 4SeT. It is argued that the competition between two opposite trends, i.e. a decrease in the base-pairing ability and an increase of the stacking interaction upon incorporation of 4SeT into a duplex, likely explains the similar experimental melting points of a seleno-derivative duplex (Se-DNA) and its native counterpart. Interestingly, the underlying electronic structure shows that replacement of O4 with Se promotes a reduction in the HOMO-LUMO gap and an increase in inter-plane coupling, which suggests that Se-DNA could be potentially useful for nanodevice applications. This finding is further supported by the fact that transfer integrals between 4SeT---A stacked base pairs are larger than those determined for similarly stacked natural T---A pairs.
2008-01-01
Ionic dissociation of chlorosulfonic acid (HSO3Cl) in the molecular clusters HSO3Cl-(H2O)n (n = 1-4) and HSO3Cl-NH3-(H2O)n (n = 0-3) was investigated by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and thermodynamic properties, such as relative enthalpy and relative Gibbs free energy, and were calculated using the hybrid density func- tional (B3LYP) method and the second order M?ller-Plesset approximation (MP2) method with the 6-311++G** basis set. Chlorosulfonic acid was found to require a minimum of three water molecules for ionization to occur and at least one water molecule to protonate ammonia. The corresponding clusters with fewer water molecules were found to be strongly hydrogen-bonded. The related properties and acid strength of chlorosulfonic acid were discussed and compared to the acid strengths of perchloric acid and sulfuric acid in the context of clusters with ammonia and water. The relative stabilities of these clusters were also investigated.
Iolanta I. Balan
2011-12-01
Full Text Available The four-stage mechanism of reaction of the rhodium trihydride complex [(triphosRhH3] (triphos=1,1,1-tris(diphenylphosphanylmethylethane with the white phosphorus molecule resulting in the phosphane and the cyclo-P3 complex [(triphosM(η3-P3] is analyzed on the basis of ab initio calculations of reactants, products, and intermediate complexes of reaction. It is shown that generation of the transient complex [(triphosRhH(η1:η1-P4] followed by intramolecular hydrogen atom migration from the metal to one of the phosphorus atoms is the energetically favourable process. Calculations also show that P4 molecule is activated by coordination to the above complex: the metal-bonded P-P edge is broken, and the tetrahedron P4 is opened to form the butterfly geometry. This activation is realized mainly due to the one-orbital back donation of 4d-electron density from the atom of Rh to the unoccupied antibonding triple degenerate t1*-MO of P4.
Roy, Soumendra K.; Jian, Tian; Lopez, Gary V.; Li, Wei-Li; Su, Jing; Bross, David H.; Peterson, Kirk A.; Wang, Lai-Sheng; Li, Jun
2016-02-01
The observation of the gaseous UFO- anion is reported, which is investigated using photoelectron spectroscopy and relativisitic ab initio calculations. Two strong photoelectron bands are observed at low binding energies due to electron detachment from the U-7sσ orbital. Numerous weak detachment bands are also observed due to the strongly correlated U-5f electrons. The electron affinity of UFO is measured to be 1.27(3) eV. High-level relativistic quantum chemical calculations have been carried out on the ground state and many low-lying excited states of UFO to help interpret the photoelectron spectra and understand the electronic structure of UFO. The ground state of UFO- is linear with an O-U-F structure and a 3H4 spectral term derived from a U 7sσ25fφ15fδ1 electron configuration, whereas the ground state of neutral UFO has a 4H7/2 spectral term with a U 7sσ15fφ15fδ1 electron configuration. Strong electron correlation effects are found in both the anionic and neutral electronic configurations. In the UFO neutral, a high density of electronic states with strong configuration mixing is observed in most of the scalar relativistic and spin-orbit coupled states. The strong electron correlation, state mixing, and spin-orbit coupling of the electronic states make the excited states of UFO very challenging for accurate quantum chemical calculations.
N2O + CO reaction over Si- and Se-doped graphenes: An ab initio DFT study
Gholizadeh, Reza; Yu, Yang-Xin
2015-12-01
Catalytic conversion of non-CO2 green house gases and other harmful gases is a promising way to protect the atmospheric environment. Non-metal atom-doped graphene is attractive for use as a catalyst in the conversion due to its unique electronic properties, relatively low price and leaving no burden to the environment. To make an attempt on the development of green catalysts for the conversion, ab initio density functional theory is used to investigate the mechanisms of N2O reduction by CO on Si- and Se-doped graphenes. We have calculated the geometries and adsorption energies of reaction species (N2O, CO, N2 and CO2) as well as energy profiles along the reaction pathways. The activation energies of N2O decomposition and CO oxidation on both Si- and Se-doped graphenes have been obtained. Our calculated results indicate that the catalytic activity of Si-doped graphene is better than the Fe+ in gas phase and comparable to the single Fe atom embedded on graphene. In the calculations, we found that van der Waals interactions and zero-point energy are two non-negligible factors for the predictions of the activation energies. Further discussion shows that Si-doped graphene can be one of efficient green catalysts for conversion of the airborne pollutants and Se-doped graphene can be a candidate for oxidizing CO by atomic oxygen.
On the stereochemical inertness of the auride lone pair: ab initio studies of AAu (A = K, Rb, Cs).
Miao, Maosheng; Brgoch, Jakoah; Krishnapriyan, Aditi; Goldman, Abby; Kurzman, Joshua A; Seshadri, Ram
2013-07-15
The "lone" 6s electron pair often plays a key role in determining the structure and physical properties of compounds containing sixth-row elements in their lower oxidation states: Tl(+), Pb(2+), and Bi(3+) with the [Xe]4f(14)5d(10)6s(2) electronic configuration. The lone pairs on these ions are associated with reduced structural symmetries, including ferroelectric instabilities and other important phenomena. Here we consider the isoelectronic auride Au(-) ion with the [Xe]4f(14)5d(10)6s(2) electronic configuration. Ab initio density functional theory methods are employed to probe the effect of the 6s lone pair in alkali-metal aurides (KAu, RbAu, and CsAu) with the CsCl structure. The dielectric constants, Born effective charges, and structural instabilities suggest that the 6s lone pair on the Au(-) anion is stereochemically inert to minor mechanical and electrical perturbation. Pressures greater than 14 GPa, however, lead to reorganization of the electronic structure of CsAu and activate lone-pair involvement and Au-Au interactions in bonding, resulting in a transformation from the cubic CsCl structure type to an orthorhombic Cmcm structure featuring zigzag Au-Au chains. PMID:23822069
Ab initio study of the structures and hydrogen storage capacity of (H2)nCH4 compound
Wang, Minghui; Cheng, Xinlu; Ren, Dahua; Zhang, Hong; Tang, Yongjian
2015-05-01
The hydrogen-rich compound (H2)nCH4 (for n = 1, 2, 3, 4) or for short (H2)nM is one of the most promising hydrogen storage materials. The (H2)4M molecule is the best hydrogen-rich compound among the (H2)nM structures and it can reach the hydrogen storage capacity of 50.2 wt.%. However, the (H2)nM always requires a certain pressure to remain stable. In this work, we first investigated the binding energy of the different structures in (H2)nM and energy barrier of H2 rotation under different pressures at ambient temperature, applying ab initio methods based on van der Waals density functional (vdW-DF). It was found that at 0 GPa, the (H2)nM is not stable, while at 5.8 GPa, the stability of (H2)nM strongly depends on its structure. We further investigate the Raman spectra of (H2)nM structures at 5.8 GPa and found the results were consistent with experiments. Excitingly, we found that boron nitride nanotubes (BNNTs) and graphite and hexagonal boron nitride (h-BN) can be used to store (H2)4M, which give insights into hydrogen storage practical applications.
Polin, Daniel; Ziegler, Joshua; Malozovsky, Yuriy; Bagayoko, Diola
We present the findings of ab-initio calculations of electronic, transport, and structural properties of cubic sodium oxide (Na2O). These results were obtained using density functional theory (DFT), specifically a local density approximation (LDA) potential, and the linear combination of Gaussian orbitals (LCGO). Our implementation of LCGO followed the Bagayoko, Zhao, and Williams method as enhanced by the work of Ekuma and Franklin (BZW-EF). We describe the electronic band structure of Na2O with a direct band gap of 2.22 eV. Our results include predicted values for the electronic band structure and associated energy eigenvalues, the total and partial density of states (DOS and pDOS), the equilibrium lattice constant of Na2O, and the bulk modulus. We have also calculated the electron and holes effective masses in the Γ to L, Γ to X, and Γ to K directions. Acknowledgments: This work was funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy - National, Nuclear Security Administration (NNSA) (Award No. DE- NA0002630), LaSPACE, and LONI-SUBR.
Brambila, Danilo S.; Harvey, Alex G.; Mašín, Zdeněk; Gorfinkiel, Jimena D.; Smirnova, Olga
2015-12-01
We present the first ab initio photoionization calculations for the NO2 molecule in its equilibrium geometry using the multichannel R-matrix method and a multiconfigurational description of the system. We focus on the role of correlation in NO2 photoionization and find that it plays a key role, both at the level of partial cross sections and asymmetry parameters. For the most sophisticated model used here, we achieve excellent agreement with the experimental data of Baltzer et al (2009 Chem. Phys. 237 451-70) for the asymmetry parameters of angle-resolved photo-electron spectra. We also present and analyse the angle-resolved photoionization dipoles for photon energies up to 90 eV and for the two lowest-energy ionization channels. Our results should advance the analysis of experiments in the field of attosecond spectroscopy, especially high harmonic generation, where angle-resolved photorecombination dipoles become crucial for the interpretation of experiments, even for randomly oriented molecular ensembles, due to coherent addition of signals from different orientations.
Ab initio study of H and He migrations in β-phase Sc, Y, and Er hydrides
Chen Ru-Cheng; Yang Li; Dai Yun-Ya; Zhu Zi-Qiang; Peng Shu-Ming; Long Xing-Gui; Gao Fei; Zu Xiao-Tao
2012-01-01
Ab initio calculations based on the density functional theory have been performed to investigate the migrations of hydrogen(H)and helium(He)atoms in β-phase scandium(Sc),yttrium(Y),and erbium(Er)hydrides with three different ratios of H to metal.The results show that the migration mechanisms of H and He atoms mainly depend on the crystal structures of hydrides,but their energy barriers are affected by the host-lattice in metal hydrides.The formation energies of octahedral-occupancy H(Hoct)and tetrahedral vacancy(Vtet)pairs are almost the same(about 1.2 eV).It is of interest to note that the migration barriers of H increase with increasing host-lattice atomic number.In addition,the results show that the favorable migration mechanism of He depends slightly on the Vtet in the Sc hydride,but strongly on that in the Y and Er hydrides,which may account for different behaviours of initial He release from ScT2 and ErT2.
The structural organization of water at a model of amorphous silica–liquid water interface is investigated by ab initio molecular dynamics (AIMD) simulations at room temperature. The amorphous surface is constructed with isolated, H-bonded vicinal and geminal silanols. In the absence of water, the silanols have orientations that depend on the local surface topology (i.e. presence of concave and convex zones). However, in the presence of liquid water, only the strong inter-silanol H-bonds are maintained, whereas the weaker ones are replaced by H-bonds formed with interfacial water molecules. All silanols are found to act as H-bond donors to water. The vicinal silanols are simultaneously found to be H-bond acceptors from water. The geminal pairs are also characterized by the formation of water H-bonded rings, which could provide special pathways for proton transfer(s) at the interface. The first water layer above the surface is overall rather disordered, with three main domains of orientations of the water molecules. We discuss the similarities and differences in the structural organization of the interfacial water layer at the surface of the amorphous silica and at the surface of the crystalline (0 0 0 1) quartz surface. (paper)
Song, Bingyi; Jiang, Wenlong; Yang, Bin; Chen, Xiumin; Xu, Baoqiang; Kong, Lingxin; Liu, Dachun; Dai, Yongnian
2016-07-01
The possibility of the separation of Pb-Sb alloys by vacuum distillation was investigated theoretically. The results show that Pb and Sb can be separated by vacuum distillation. However, the experimental results show that vacuum distillation technique does not provide clear separation. According to the literature, Pb-Sb alloys belong to azeotropic compounds under some certain temperature; the experiment and computer simulation were carried out based on the exceptional condition so as to analyze the reason from the experiment and microstructure of Pb-Sb alloys perspective. The separation of Pb-Sb alloys by vacuum distillation was experimentally carried out to probe the azeotropic point. Also, the functions, such as partial radial distributions functions, the structure factor, mean square displacement, and the density of state, were calculated by ab-initio molecular dynamics for the representation of the structure and properties of Pb-Sb alloys with different composition of Sb. The experimental results indicate that there exists common volatilization for Pb-Sb alloys when Sb content is 16.5 wt pct. On the other hand, the calculation results show that there is an intense interaction between Pb and Sb when Sb content is 22 wt pct, which supports the experimental results although Sb content is slightly deviation.
Bryk, Taras; Wax, J.-F.
2016-05-01
Using a combination of ab initio molecular dynamics and several fit models for dynamic structure of liquid metals, we explore an issue of possible manifestation of non-acoustic collective excitations in longitudinal dynamics having liquid Na as a case study. A model with two damped harmonic oscillators (DHOs) in time domain is used for analysis of the density-density time correlation functions. Another similar model with two propagating contributions and three lowest exact sum rules is considered, as well as an extended hydrodynamic model known as thermo-viscoelastic one which permits two types of propagating modes outside the hydrodynamic region to be used for comparison with ab initio obtained time correlation functions and calculations of dispersions of collective excitations. Our results do not support recent suggestions that, even in simple liquid metals, non-hydrodynamics transverse excitations contribute to the longitudinal collective dynamics and can be detected as a DHO-like spectral shape at their transverse frequency. We found that the thermo-viscoelastic dynamic model permits perfect description of the density-density and current-current time correlation functions of the liquid Na in a wide range of wave numbers, which implies that the origin of the non-hydrodynamic collective excitations contributing to longitudinal dynamics can be short-wavelength heat waves.
Barrett, B R; Navratil, P; Vary, J P
2011-04-11
A long-standing goal of nuclear theory is to determine the properties of atomic nuclei based on the fundamental interactions among the protons and neutrons (i.e., nucleons). By adopting nucleon-nucleon (NN), three-nucleon (NNN) and higher-nucleon interactions determined from either meson-exchange theory or QCD, with couplings fixed by few-body systems, we preserve the predictive power of nuclear theory. This foundation enables tests of nature's fundamental symmetries and offers new vistas for the full range of complex nuclear phenomena. Basic questions that drive our quest for a microscopic predictive theory of nuclear phenomena include: (1) What controls nuclear saturation; (2) How the nuclear shell model emerges from the underlying theory; (3) What are the properties of nuclei with extreme neutron/proton ratios; (4) Can we predict useful cross sections that cannot be measured; (5) Can nuclei provide precision tests of the fundamental laws of nature; and (6) Under what conditions do we need QCD to describe nuclear structure, among others. Along with other ab initio nuclear theory groups, we have pursued these questions with meson-theoretical NN interactions, such as CD-Bonn and Argonne V18, that were tuned to provide high-quality descriptions of the NN scattering phase shifts and deuteron properties. We then add meson-theoretic NNN interactions such as the Tucson-Melbourne or Urbana IX interactions. More recently, we have adopted realistic NN and NNN interactions with ties to QCD. Chiral perturbation theory within effective field theory ({chi}EFT) provides us with a promising bridge between QCD and hadronic systems. In this approach one works consistently with systems of increasing nucleon number and makes use of the explicit and spontaneous breaking of chiral symmetry to expand the strong interaction in terms of a dimensionless constant, the ratio of a generic small momentum divided by the chiral symmetry breaking scale taken to be about 1 GeV/c. The
A long-standing goal of nuclear theory is to determine the properties of atomic nuclei based on the fundamental interactions among the protons and neutrons (i.e., nucleons). By adopting nucleon-nucleon (NN), three-nucleon (NNN) and higher-nucleon interactions determined from either meson-exchange theory or QCD, with couplings fixed by few-body systems, we preserve the predictive power of nuclear theory. This foundation enables tests of nature's fundamental symmetries and offers new vistas for the full range of complex nuclear phenomena. Basic questions that drive our quest for a microscopic predictive theory of nuclear phenomena include: (1) What controls nuclear saturation; (2) How the nuclear shell model emerges from the underlying theory; (3) What are the properties of nuclei with extreme neutron/proton ratios; (4) Can we predict useful cross sections that cannot be measured; (5) Can nuclei provide precision tests of the fundamental laws of nature; and (6) Under what conditions do we need QCD to describe nuclear structure, among others. Along with other ab initio nuclear theory groups, we have pursued these questions with meson-theoretical NN interactions, such as CD-Bonn and Argonne V18, that were tuned to provide high-quality descriptions of the NN scattering phase shifts and deuteron properties. We then add meson-theoretic NNN interactions such as the Tucson-Melbourne or Urbana IX interactions. More recently, we have adopted realistic NN and NNN interactions with ties to QCD. Chiral perturbation theory within effective field theory (χEFT) provides us with a promising bridge between QCD and hadronic systems. In this approach one works consistently with systems of increasing nucleon number and makes use of the explicit and spontaneous breaking of chiral symmetry to expand the strong interaction in terms of a dimensionless constant, the ratio of a generic small momentum divided by the chiral symmetry breaking scale taken to be about 1 GeV/c. The resulting NN
Ab initio simulation of transport phenomena in rarefied gases.
Sharipov, Felix; Strapasson, José L
2012-09-01
Ab initio potentials are implemented into the direct simulation Monte Carlo (DSMC) method. Such an implementation allows us to model transport phenomena in rarefied gases without any fitting parameter of intermolecular collisions usually extracted from experimental data. Applying the method proposed by Sharipov and Strapasson [Phys. Fluids 24, 011703 (2012)], the use of ab initio potentials in the DSMC requires the same computational efforts as the widely used potentials such as hard spheres, variable hard sphere, variable soft spheres, etc. At the same time, the ab initio potentials provide more reliable results than any other one. As an example, the transport coefficients of a binary mixture He-Ar, viz., viscosity, thermal conductivity, and thermal diffusion factor, have been calculated for several values of the mole fraction. PMID:23030889
Londero, Elisa; Bijeikytė, Monika; Maze, Jeromino R; Alkauskas, Audrius; Gali, Adam
2016-01-01
Understanding optical excitation spectra of point defects is still a scientific challenge. We demonstrate by \\emph{ab initio} calculations that a prominent sharp feature in the photoluminescence (PL) spectrum of the negatively charged silicon-vacancy defect in diamond can be only explained within the Herzberg-Teller approximation that goes beyond the commonly applied Franck-Condon approximation. The effect of the dynamic Jahn-Teller effect on the PL spectrum is also discussed. Our implementation of Herzberg-Teller theory paves the way for full \\emph{ab initio} description of the optical excitation spectrum of point defects in solids
Chen, Shuang; Xiong, Wei; Zhou, Yun Shen; Lu, Yong Feng; Zeng, Xiao Cheng
2016-05-01
Ab initio molecular dynamics (AIMD) simulations are employed to investigate the chemical mechanism underlying the Ni-catalyzed transformation of amorphous carbon (a-C) into graphene in the rapid thermal processing (RTP) experiment to directly grow graphene on various dielectric surfaces via the evaporation of surplus Ni and C at 1100 °C (below the melting point of bulk Ni). It is found that the a-C-to-graphene transformation entails the metal-induced crystallization and layer exchange mechanism, rather than the conventional dissolution/precipitation mechanism typically involved in Ni-catalyzed chemical vapor deposition (CVD) growth of graphene. The multi-layer graphene can be tuned by changing the relative thicknesses of deposited a-C and Ni thin films. Our AIMD simulations suggest that the easy evaporation of surplus Ni with excess C is likely attributed to the formation of a viscous-liquid-like Ni-C solution within the temperature range of 900-1800 K and to the faster diffusion of C atoms than that of Ni atoms above 600 K. Even at room temperature, sp3-C atoms in a-C are quickly converted to sp2-C atoms in the course of the simulation, and the graphitic C formation can occur at low temperature. When the temperature is as high as 1200 K, the grown graphitic structures reversely dissolve into Ni. Because the rate of temperature increase is considerably faster in the AIMD simulations than in realistic experiments, defects in the grown graphitic structures are kinetically trapped. In this kinetic growth stage, the carbon structures grown from sp3-carbon or from sp2-carbon exhibit marked differences.Ab initio molecular dynamics (AIMD) simulations are employed to investigate the chemical mechanism underlying the Ni-catalyzed transformation of amorphous carbon (a-C) into graphene in the rapid thermal processing (RTP) experiment to directly grow graphene on various dielectric surfaces via the evaporation of surplus Ni and C at 1100 °C (below the melting point of bulk
Valdiviés-Cruz, Karell; Lam, Anabel; Zicovich-Wilson, Claudio M
2015-09-28
Periodic quantum-chemistry methods as implemented in the CRYSTAL14 code were considered to analyse the interaction of acid clinoptilolite with water. Initially adsorbed molecules hydrolyse the Al-O bonds, giving rise to defective dealuminated materials. A suitable and representative periodic model of the partially disordered hydrated H-zeolite is the primitive cell (18 T sites) of a decahydrated trialuminated structure of HEU topology. The water distribution inside the material cavities was initially investigated. The model considered for further dealumination was the most stable one from those generated through a combined force field Monte Carlo and ab initio optimization strategy. Optimizations and energy estimations were made at the hybrid DFT level of theory (PBE0 functional) with an atomic basis set of VDZP quality. The energetics of the different pathways involved in the dealumination process was addressed by considering the Gibbs free energy with thermal and zero-point corrections through phonon analysis. It arises that hydrated models exhibit protonated water clusters stabilized by different kinds of H-bonds. The first Al extraction is slightly more energetically favourable from T3 than T2 sites, but at the same time the latter is more probable owing to its larger Al population. However, concerning the second dealumination step, it is more favourable removing the Al atom from both remaining sites after a starting abstraction from T2 rather than T3. These facts determine that the most probable overall pathways go through a first Al removal from T2. The agreement with experimental results is discussed. PMID:26299763
Chen, Shuang; Xiong, Wei; Zhou, Yun Shen; Lu, Yong Feng; Zeng, Xiao Cheng
2016-05-14
Ab initio molecular dynamics (AIMD) simulations are employed to investigate the chemical mechanism underlying the Ni-catalyzed transformation of amorphous carbon (a-C) into graphene in the rapid thermal processing (RTP) experiment to directly grow graphene on various dielectric surfaces via the evaporation of surplus Ni and C at 1100 °C (below the melting point of bulk Ni). It is found that the a-C-to-graphene transformation entails the metal-induced crystallization and layer exchange mechanism, rather than the conventional dissolution/precipitation mechanism typically involved in Ni-catalyzed chemical vapor deposition (CVD) growth of graphene. The multi-layer graphene can be tuned by changing the relative thicknesses of deposited a-C and Ni thin films. Our AIMD simulations suggest that the easy evaporation of surplus Ni with excess C is likely attributed to the formation of a viscous-liquid-like Ni-C solution within the temperature range of 900-1800 K and to the faster diffusion of C atoms than that of Ni atoms above 600 K. Even at room temperature, sp(3)-C atoms in a-C are quickly converted to sp(2)-C atoms in the course of the simulation, and the graphitic C formation can occur at low temperature. When the temperature is as high as 1200 K, the grown graphitic structures reversely dissolve into Ni. Because the rate of temperature increase is considerably faster in the AIMD simulations than in realistic experiments, defects in the grown graphitic structures are kinetically trapped. In this kinetic growth stage, the carbon structures grown from sp(3)-carbon or from sp(2)-carbon exhibit marked differences. PMID:27117235
周立新
2000-01-01
Results or ab initio selr-consistent-field ( SCF) and denityfunctional theory (DFr) calculations of the gas-phase structure, acidity (free energy of deprotona tion, △G°) and aroma ticity of tetraselenosquaric acid (3, 4-diseleny-3-cyclobutene-1,2-diselenone, H2C4Se4) are reported.The global minimu found on the potenial energy surface of tetraselenosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE is omers are very close in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutenediselenol. The computed aromaic stabilization energy(ASE)by homodesmotic reaction is -77.4 (MP2(fu)/6 - 311 + G* *//RHF/6 - 311 + G* * ) and - 54.8 kJ/mol (B3LYP/6 - 311 + G* * //B3LYP/6 - 311 + G* * ). The aromaticity of tetraselenosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT(IGAIM) - RHF/6 - 311 + G**//RHF/6 - 311 + G* * and - 32.91(4π· 10-6 m3/mo l)(CSGT(IGA1M)-B3LYP/6 - 311 + G* *//B3LYP/6 - 311 + G* * ).Thus, tetraselenosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated gas-phase acidity is △G10(298K) = 1257.7 and △G20(298K) = 1617.1 kJ/mol. Hence, tetraselenosquarc acid is the stronest acid among the three squaric acids (3,4-dihydroxy-3-cyclobutene-1,2-dione, H2C4O4, 3,4-dithiohydroxy-3- cyclobutene-1,2-dithione, H2C4S4, 3,4- diselenyl-3- cyclobutene- 1,2-diselenone, H2C4Se4).
Roy, Soumendra K; Jian, Tian; Lopez, Gary V; Li, Wei-Li; Su, Jing; Bross, David H; Peterson, Kirk A; Wang, Lai-Sheng; Li, Jun
2016-02-28
The observation of the gaseous UFO(-) anion is reported, which is investigated using photoelectron spectroscopy and relativisitic ab initio calculations. Two strong photoelectron bands are observed at low binding energies due to electron detachment from the U-7sσ orbital. Numerous weak detachment bands are also observed due to the strongly correlated U-5f electrons. The electron affinity of UFO is measured to be 1.27(3) eV. High-level relativistic quantum chemical calculations have been carried out on the ground state and many low-lying excited states of UFO to help interpret the photoelectron spectra and understand the electronic structure of UFO. The ground state of UFO(-) is linear with an O-U-F structure and a (3)H4 spectral term derived from a U 7sσ(2)5fφ(1)5fδ(1) electron configuration, whereas the ground state of neutral UFO has a (4)H(7/2) spectral term with a U 7sσ(1)5fφ(1)5fδ(1) electron configuration. Strong electron correlation effects are found in both the anionic and neutral electronic configurations. In the UFO neutral, a high density of electronic states with strong configuration mixing is observed in most of the scalar relativistic and spin-orbit coupled states. The strong electron correlation, state mixing, and spin-orbit coupling of the electronic states make the excited states of UFO very challenging for accurate quantum chemical calculations. PMID:26931704
Joshi, Prasad Ramesh; Ramanathan, N; Sundararajan, K; Sankaran, K
2015-04-01
The weak interaction between PCl3 and CH3OH was investigated using matrix isolation infrared spectroscopy and ab initio computations. In a nitrogen matrix at low temperature, the noncovalent adduct was generated and characterized using Fourier transform infrared spectroscopy. Computations were performed at B3LYP/6-311++G(d,p), B3LYP/aug-cc-pVDZ, and MP2/6-311++G(d,p) levels of theory to optimize the possible geometries of PCl3-CH3OH adducts. Computations revealed two minima on the potential energy surface, of which, the global minimum is stabilized by a noncovalent P···O interaction, known as a pnictogen bonding (phosphorus bonding or P-bonding). The local minimum corresponded to a cyclic adduct, stabilized by the conventional hydrogen bonding (Cl···H-O and Cl···H-C interactions). Experimentally, 1:1 P-bonded PCl3-CH3OH adduct in nitrogen matrix was identified, where shifts in the P-Cl modes of PCl3, O-C, and O-H modes of CH3OH submolecules were observed. The observed vibrational frequencies of the P-bonded adduct in a nitrogen matrix agreed well with the computed frequencies. Furthermore, computations also predicted that the P-bonded adduct is stronger than H-bonded adduct by ∼1.56 kcal/mol. Atoms in molecules and natural bond orbital analyses were performed to understand the nature of interactions and effect of charge transfer interaction on the stability of the adducts. PMID:25772403
Xin, Yan; Hou, S. C.; Xiang, Lan; Yu, Yang-Xin
2015-12-01
Calcium sulfate hemihydrate (CaSO4·0.5H2O, CSH) whiskers with high aspect ratio are promising reinforce materials which have drawn much attention. In order to obtain high quality CSH materials, effect of Mg2+ ions on properties of the (0 0 2), (2 0 0)1 and (2 0 0)2 planes of CSH is investigated using an ab initio density functional theory (DFT) with a van der Waals (vdW) dispersion-correction. The computed results show that strong adsorption and substitution effects take place between Mg2+ ion and (2 0 0)1 plane. The adsorption energies of an Mg2+ ion on the (0 0 2), (2 0 0)1 and (2 0 0)2 planes are -0.066, -0.571 and -0.047 eV, respectively. An insight into the electrostatic potential of pristine CSH planes has demonstrated that the (2 0 0)1 plane is much more negatively charged than the (0 0 2) and (2 0 0)2 planes. The energies of the substitution of a Ca atom with an Mg atom on the CSH (0 0 2), (2 0 0)1 and (2 0 0)2 planes are 1.572, 0.063 and 1.349 eV, respectively. It is found that Ca atoms on the (2 0 0)1 plane are relatively easy to be substituted by Mg atoms. The calculation results of a Ca2+ ion adsorption on the Mg-doped (2 0 0)1 plane indicate that the adsorption energies increase apparently as the doping ratio varies from 0 to 1.0. Compared with K+, Na+ and Al3+ ions, Mg2+ ion is the most promising additive to promote the growth of CSH along c axis.
Rotational study of the CH4–CO complex: Millimeter-wave measurements and ab initio calculations
The rotational spectrum of the van der Waals complex CH4–CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110–145 GHz. Newly observed and assigned transitions belong to the K = 2–1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2–1 and K = 0–1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4–CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4–CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm−1. The bound rovibrational levels of the CH4–CO complex were calculated for total angular momentum J = 0–6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm−1 for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4–CO, respectively
Towards new horizons in ab initio nuclear structure theory
We review recent advances in ab initio nuclear structure theory, which have changed the horizons of this field. Starting from chiral effective field theory to construct the nuclear Hamiltonian and the similarity renormalization group to further soften it, we address several many-body approaches that have seen major developments over the past few years. We show that the domain of ab initio nuclear structure theory has been pushed well beyond the p-shell and that quantitative QCD-based predictions are becoming possible all the way from the proton to the neutron drip line up into the medium-mass regime. (authors)
Use of ab initio quantum chemical methods in battery technology
Deiss, E. [Paul Scherrer Inst. (PSI), Villigen (Switzerland)
1997-06-01
Ab initio quantum chemistry can nowadays predict physical and chemical properties of molecules and solids. An attempt should be made to use this tool more widely for predicting technologically favourable materials. To demonstrate the use of ab initio quantum chemistry in battery technology, the theoretical energy density (energy per volume of active electrode material) and specific energy (energy per mass of active electrode material) of a rechargeable lithium-ion battery consisting of a graphite electrode and a nickel oxide electrode has been calculated with this method. (author) 1 fig., 1 tab., 7 refs.
P-V Relation for Mercuric Calcogenides: Ab Initio Method
G. Misra
2011-01-01
Full Text Available Mercuric Calcogenides found many applications in electronic and optical devices as semiconducting materials. An equation of state provides useful information about the relationship between pressure (P, volume (V and temperature (T that helps to understand the behaviour of materials under the effect of high pressure and high temperature. The present paper sheds light on the electronic structure of Mercuric Calcogenides by simulating its electronic properties through ab initio method. This ab initio method is extended to derive the equation of state for Mercuric Calcogenides. The present equation of state has also been tested for the prediction of End Point. The computed results compare well with Quantum statistical data.
Recent achievements in ab initio modelling of liquid water
Khaliullin, Rustam Z
2013-01-01
The application of newly developed first-principle modeling techniques to liquid water deepens our understanding of the microscopic origins of its unusual macroscopic properties and behaviour. Here, we review two novel ab initio computational methods: second-generation Car-Parrinello molecular dynamics and decomposition analysis based on absolutely localized molecular orbitals. We show that these two methods in combination not only enable ab initio molecular dynamics simulations on previously inaccessible time and length scales, but also provide unprecedented insights into the nature of hydrogen bonding between water molecules. We discuss recent applications of these methods to water clusters and bulk water.
Jaiganesh, G.; Jaya, S. Mathi
2015-06-01
The magnetism, structure and spin polarized electronic structure of Ti substituted MO (M = Mg, Ca, Sr) are studied using the ab-initio techniques within the framework of the density functional theory. Appropriately constructed supercell along with the full structural optimization of these cells is used for studying the influence of Ti substitution on the magnetism and electronic structure of these compounds. We find from our calculations that the Ti substituted MO compounds energetically favor magnetically ordered state. The Ti concentration is found to be important in deciding the magnetic order and we have observed antiferromagnetic order for the Ti concentration of 0.25. The Ti substituted MO compounds are thus an interesting class of materials that deserve further studies.
The magnetism, structure and spin polarized electronic structure of Ti substituted MO (M = Mg, Ca, Sr) are studied using the ab-initio techniques within the framework of the density functional theory. Appropriately constructed supercell along with the full structural optimization of these cells is used for studying the influence of Ti substitution on the magnetism and electronic structure of these compounds. We find from our calculations that the Ti substituted MO compounds energetically favor magnetically ordered state. The Ti concentration is found to be important in deciding the magnetic order and we have observed antiferromagnetic order for the Ti concentration of 0.25. The Ti substituted MO compounds are thus an interesting class of materials that deserve further studies
Masrour, R., E-mail: rachidmasrour@hotmail.com [Laboratory of Materials, Processes, Environment and Quality, Cady Ayyed University, National School of Applied Sciences, BP. 63, 46000 Safi (Morocco); LMPHE (URAC 12), Faculty of Science, Mohammed V-Agdal University, Rabat (Morocco); Hlil, E.K. [Institut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9 (France); Hamedoun, M. [Institute of Nanomaterials and Nanotechnologies, MAScIR, Rabat (Morocco); Benyoussef, A. [LMPHE (URAC 12), Faculty of Science, Mohammed V-Agdal University, Rabat (Morocco); Institute of Nanomaterials and Nanotechnologies, MAScIR, Rabat (Morocco); Hassan II Academy of Science and Technology, Rabat (Morocco); Mounkachi, O.; El Moussaoui, H. [Institute of Nanomaterials and Nanotechnologies, MAScIR, Rabat (Morocco)
2014-06-01
Self-consistent ab initio calculations, based on DFT (Density Functional Theory) approach and using FLAPW (Full potential Linear Augmented Plane Wave) method, are performed to investigate both electronic and magnetic properties of the MnSe lattice. Polarized spin and spin–orbit coupling are included in calculations within the framework of the antiferromagnetic state between two adjacent Mn lattices. Magnetic moments considered to lie along (001) axes are computed. Obtained data from ab initio calculations are used as input for the high temperature series expansions (HTSEs) calculations to compute other magnetic parameters. The zero-field high temperature static susceptibility series of the spin −4.28 nearest-neighbor Ising model on face centered cubic (fcc) and lattices is thoroughly analyzed by means of a power series coherent anomaly method (CAM). The exchange interaction between the magnetic atoms and the Néel temperature are deduced using the mean filed and HTSEs theories. - Highlights: • Ab initio calculations are used to investigate both electronic and magnetic properties of the MnSe alloys. • Obtained data from ab initio calculations are used as input for the HTSEs. • The Néel temperature is obtained for MnSe alloys.
Chalupský, Jakub; Rokob, Tibor András; Kurashige, Y.; Yanai, T.; Solomon, E. I.; Rulíšek, Lubomír; Srnec, Martin
2014-01-01
Roč. 136, č. 45 (2014), s. 15977-15991. ISSN 0002-7863 R&D Projects: GA ČR(CZ) GA14-31419S Institutional support: RVO:61388963 ; RVO:61388955 Keywords : DMRG-CASPT2 * ab initio calculations * reaction mechanisms Subject RIV: CF - Physical ; Theoretical Chemistry Impact factor: 12.113, year: 2014
Dahmoune, C.; Lounis, S.; Talanana, M.; Benakki, M.; Bouarab, S.; Demangeat, C.
2002-01-01
Ab initio calculations of the local spin polarization at the (0 0 1) surfaces performed on the binary FePd and FeRh alloys are presented. For Rh-terminated FeRh (0 0 1) surface, the calculations indicate a possible magnetic reconstruction leading to a ferromagnetic order in the surface region, in co
Souvatzis, P.; Legut, Dominik; Eriksson, M.; Katsnelson, M.I.
2010-01-01
Roč. 81, č. 9 (2010), 092201/1-092201/4. ISSN 1098-0121 Institutional research plan: CEZ:AV0Z20410507 Keywords : NiTi * ab initio * phonon * anharmonicity * stabilization of beta phase Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 3.772, year: 2010
Pederzoli, Marek; Pittner, Jiří; Barbatti, M.; Lischka, H.
Bellingham: SPIE-INT SOC OPTICAL ENGINEERING, 2012 - (Dobisz, E.; Eldada, L.), s. 846318 ISBN 978-0-8194-9180-0. [Conference on Nanoengineering - Fabrication, Properties, Optics, and Devices /9./. San Diego (US), 14.08.2012-16.08.2012] Institutional support: RVO:61388955 Keywords : azobenzene * ab initio molecular dynamics * excited states Subject RIV: CF - Physical ; Theoretical Chemistry
Extensive ab initio study of the electronic states of S2 molecule including spin-orbit coupling
Xing, Wei; Shi, Deheng; Sun, Jinfeng; Liu, Hui; Zhu, Zunlue
2013-03-01
The potential energy curves (PECs) of 15 Λ-S states and 24 Ω states generated from the 13 Λ-S bound states of the S2 molecule are investigated in detail using an ab initio quantum chemical method. The PECs are calculated for internuclear separations from 0.12 to 1.10 nm by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification (MRCI + Q). The spin-orbit (SO) coupling effect is accounted for by the Breit-Pauli Hamiltonian. To discuss the effect on the energy splitting by the core-electron correlations, the all-electron basis set, cc-pCVTZ with and without 2s2p correlations, is used for the SO coupling calculations of the A3 ? and B‧3Πg Λ-S states since their measurements can be found in the literature. By comparison, the cc-pCVTZ basis set with 2s2p correlations is chosen for the SO coupling calculations of 13 Λ-S bound states. To improve the quality of PECs, core-valence correlation and scalar relativistic corrections are included. Scalar relativistic correction calculations are made using the third-order Douglas-Kroll Hamiltonian (DKH3) approximation at the level of a cc-pV5Z basis set. Core-valence correlation corrections are taken into account with a cc-pCVTZ basis set. The spectroscopic parameters of 13 Λ-S bound states and 24 Ω states are calculated. With the PECs obtained by the MRCI + Q/aug-cc-pV6Z + CV + DK + SO calculations, the SO coupling splitting energies are 379.25 cm-1 between the A‧3 and A‧2 Ω state, 83.40 cm-1 between the A1 and A0- Ω state and 210.91 cm-1 between the B‧2 and B‧1 Ω state, which agree well with the corresponding measurements of 383, 77.51 and 209 cm-1, respectively. Moreover, other spectroscopic parameters are also in excellent agreement with the measurements. It demonstrates that the spectroscopic parameters of 24 Ω states reported here for the first time can be expected to be
Jiang, Lei; Xu, Yi-sheng; Ding, Ai-zhong
2010-12-01
The mechanism of the reaction of the sulfur dioxide (SO(2)) with four stabilized Criegee intermediates (stabCI-CH(3)-OO, stabCI-OO, stabCIx-OO, and stabCH(2)OO) produced via the ozonolysis of limonene have been investigated using ab initio and DFT (density functional theory) methods. It has been shown that the intermediate adduct formed by the initiation of these reactions may be followed by two different reaction pathways such as H migration reaction to form carboxylic acids and rearrangement of oxygen to produce the sulfur trioxide (SO(3)) from the terminal oxygen of the COO group and SO(2). We found that the reaction of stabCI-OO and stabCH(2)OO with SO(2) can occur via both the aforementioned scenarios, whereas that of stabCI-CH(3)-OO and stabCIx-OO with SO(2) is limited to the second pathway only due to the absence of migrating H atoms. It has been shown that at the CCSD(T)/6-31G(d) + CF level of theory the activation energies of six reaction pathways are in the range of 14.18-22.59 kcal mol(-1), with the reaction between stabCIx-OO and SO(2) as the most favorable pathway of 14.18 kcal mol(-1) activation energy and that the reaction of stabCI-OO and stabCH(2)OO with SO(2) occurs mainly via the second reaction path. The thermochemical analysis of the reaction between SO(2) and stabilized Criegee intermediates indicates that the reaction of SO(2) and stabilized Criegee intermediates formed from the exocyclic primary ozonide decomposition is the main pathway of the SO(3) formation. This is likely to explain the large (~100%) difference in the production rate in the favor of the exocyclic compounds observed in recent experiments on the formation of H(2)SO(4) from exocyclic and endocyclic compounds. PMID:21053959
Rotational study of the NH3–CO complex: Millimeter-wave measurements and ab initio calculations
The rotational spectrum of the van der Waals complex NH3–CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 112–139 GHz. Newly observed and assigned transitions belong to the K = 0–0, K = 1–1, K = 1–0, and K = 2–1 subbands correlating with the rotationless (jk)NH3 = 00 ground state of free ortho-NH3 and the K = 0–1 and K = 2–1 subbands correlating with the (jk)NH3 = 11 ground state of free para-NH3. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. Some of these transitions are continuations to higher J values of transition series observed previously [C. Xia et al., Mol. Phys. 99, 643 (2001)], the other transitions constitute newly detected subbands. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the ortho-NH3–CO and para-NH3–CO complexes. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of NH3–CO has been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations and an augmented correlation-consistent triple zeta basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the N atom closest to the CO subunit and binding energy De = 359.21 cm−1. The bound rovibrational levels of the NH3–CO complex were calculated for total angular momentum J = 0–6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 210.43 and 218.66 cm−1 for ortho-NH3–CO and para-NH3–CO, respectively
Saheer, V. C.; Kumar, Sanjay
2016-01-01
The global ground and first three excited electronic state adiabatic as well as the corresponding quasidiabatic potential energy surfaces is reported as a function of nuclear geometries in the Jacobi coordinates ( R → , r → , γ ) using Dunning's cc-pVTZ basis set at the internally contracted multi-reference (single and double) configuration interaction level of accuracy. Nonadiabatic couplings, arising out of relative motion of proton and the vibrational motion of CO, are also reported in terms of coupling potentials. The quasidiabatic potential energy surfaces and the coupling potentials have been obtained using the ab initio procedure [Simah et al., J. Chem. Phys. 111, 4523 (1999)] for the purpose of dynamics studies.
Ali Niazi; Saeed Jameh-Bozorghi; Davood Nori-Shargh
2007-01-01
A quantitative structure-activity relationships (QSAR) study is suggested for the prediction of solubility of some thiazolidine-4-carboxylic acid derivatives in aqueous solution. Ab initio theory was used to calculate some quantum chemical descriptors including electrostatic potentials and local charges at each atom, HOMO and LUMO energies, etc. Modeling of the solubility of thiazolidine4-carboxylic acid derivatives as a function of molecular structures was established by means of the partial least squares (PLS). The subset of descriptors, which resulted in the low prediction error, was selected by genetic algorithm. This model was applied for the prediction of the solubility of some thiazolidine-4-carboxylic acid derivatives, which were not in the modeling procedure. The relative errors of prediction lower that -4% was obtained by using GA-PLS method. The resulted model showed high prediction ability with RMSEP of 3.8836 and 2.9500 for PLS and GA-PLS models, respectively.
An ab initio study within the density functional theory of the recently described reversible hydrogen storage reaction Mg(NH2)2 + 2LiH ↔ Li2Mg(NH)2 + 2H2 has been conducted. The electronic structure, structural parameters, vibrational spectra, and enthalpies of formation of all the reactants and products as well as the heat of the overall reaction at zero and finite temperature have been calculated in the generalized gradient approximation (GGA) to the exchange correlation potential. The heat of the overall reaction is calculated to be 53.4 kJ/mol H2 in contrast to the experimentally obtained overall heat of reaction of ∼44.1 kJ/mol H2. The difference of ∼20% between the experimental and calculated values is discussed
Ab initio calculations of mechanical properties: Methods and applications
Pokluda, J.; Černý, Miroslav; Šob, Mojmír; Umeno, Y.
2015-01-01
Roč. 73, AUG (2015), s. 127-158. ISSN 0079-6425 R&D Projects: GA ČR(CZ) GAP108/12/0311 Institutional support: RVO:68081723 Keywords : Ab initio methods * Elastic moduli * Intrinsic hardness * Stability analysis * Theoretical strength * Intrinsic brittleness/ductility Subject RIV: BM - Solid Matter Physics ; Magnetism Impact factor: 27.417, year: 2014
Cyanogen Azide. Ionization Potentials and Ab Initio SCF MO Calculation
Bak, Börge; Jansen, Peter; Stafast, Herbert
1975-01-01
The Ne(I) and He(I) photoelectron(PE) spectra of cyanogen azide, NCN3, have been recorded at high resolution. Their interpretation is achieved by comparison with the PE spectrum of HN3 and an ab initio LCGO SCF MO calculation. Deviations from Koopmans' theorem of quite different magnitudes are fo...
The F2BO free radical is a known, although little studied, species but similar X2BY (X = H, D, F; Y = O, S) molecules are largely unknown. High level ab initio methods have been used to predict the molecular structures, vibrational frequencies (in cm−1), and relative energies of the ground and first two excited electronic states of these free radicals, as an aid to their eventual spectroscopic identification. The chosen theoretical methods and basis sets were tested on F2BO and found to give good agreement with the known experimental quantities. In particular, complete basis set extrapolations of coupled-cluster single and doubles with perturbative triple excitations/aug-cc-pVXZ (X = 3, 4, 5) energies gave excellent electronic term values, due to small changes in geometry between states and the lack of significant multireference character in the wavefunctions. The radicals are found to have planar C2v geometries in the X~2B2 ground state, the low-lying A~2B1 first excited state, and the higher B~2A1 state. Some of these radicals have very small ground state dipole moments hindering microwave measurements. Infrared studies in matrices or in the gas phase may be possible although the fundamentals of H2BO and H2BS are quite weak. The most promising method of identifying these species in the gas phase appears to be absorption or laser-induced fluorescence spectroscopy through the allowed B~-X~ transitions which occur in the visible-near UV region of the electromagnetic spectrum. The ab initio results have been used to calculate the Franck-Condon profiles of the absorption and emission spectra, and the rotational structure of the B~-X~000 bands has been simulated. The calculated single vibronic level emission spectra provide a unique, readily recognizable fingerprint of each particular radical, facilitating the experimental identification of new X2BY species in the gas phase
Ab initio calculations of grain boundaries in bcc metals
Scheiber, Daniel; Pippan, Reinhard; Puschnig, Peter; Romaner, Lorenz
2016-03-01
In this study, we compute grain boundary (GB) properties for a large set of GBs in bcc transition metals with a special focus on W, Mo and Fe using ab initio density functional theory (DFT) and semi-empirical second nearest neighbour modified embedded atom method (2NN-MEAM) potentials. The GB properties include GB energies, surface energies, GB excess volume and work of separation, which we analyse and then compare to experimental data. We find that the used 2NN-MEAM potentials can predict general trends of GB properties, but do not always reproduce the GB ground state structure and energy found with DFT. In particular, our results explain the experimental finding that W and Mo prefer intergranular fracture, while other bcc metals prefer transgranular cleavage.
Interatomic Coulombic decay widths of helium trimer: Ab initio calculations
Kolorenč, Přemysl, E-mail: kolorenc@mbox.troja.mff.cuni.cz [Charles University in Prague, Faculty of Mathematics and Physics, Institute of Theoretical Physics, V Holešovičkách 2, 180 00 Prague (Czech Republic); Sisourat, Nicolas [Sorbonne Universités, UPMC Univ Paris 06, UMR 7614, Laboratoire de Chimie Physique Matière et Rayonnement, F-75005 Paris (France); CNRS, UMR 7614, Laboratoire de Chimie Physique Matière et Rayonnement, F-75005 Paris (France)
2015-12-14
We report on an extensive study of interatomic Coulombic decay (ICD) widths in helium trimer computed using a fully ab initio method based on the Fano theory of resonances. Algebraic diagrammatic construction for one-particle Green’s function is utilized for the solution of the many-electron problem. An advanced and universal approach to partitioning of the configuration space into discrete states and continuum subspaces is described and employed. Total decay widths are presented for all ICD-active states of the trimer characterized by one-site ionization and additional excitation of an electron into the second shell. Selected partial decay widths are analyzed in detail, showing how three-body effects can qualitatively change the character of certain relaxation transitions. Previously unreported type of three-electron decay processes is identified in one class of the metastable states.
Li, Wu
2015-08-01
We demonstrate the ab initio electrical transport calculation limited by electron-phonon coupling by using the full solution of the Boltzmann transport equation (BTE), which applies equally to metals and semiconductors. Numerical issues are emphasized in this work. We show that the simple linear interpolation of the electron-phonon coupling matrix elements from a relatively coarse grid to an extremely fine grid can ease the calculational burden, which makes the calculation feasible in practice. For the Brillouin zone (BZ) integration of the transition probabilities involving one δ function, the Gaussian smearing method with a physical choice of locally adaptive broadening parameters is employed. We validate the calculation in the cases of n -type Si and Al. The calculated conductivity and mobility are in good agreement with experiments. In the metal case we also demonstrate that the Gaussian smearing method with locally adaptive broadening parameters works excellently for the BZ integration with double δ functions involved in the Eliashberg spectral function and its transport variant. The simpler implementation is the advantage of the Gaussian smearing method over the tetrahedron method. The accuracy of the relaxation time approximation and the approximation made by Allen [Phys. Rev. B 17, 3725 (1978), 10.1103/PhysRevB.17.3725] has been examined by comparing with the exact solution of BTE. We also apply our method to n -type monolayer MoS2, for which a mobility of 150 cm2 v-1 s-1 is obtained at room temperature. Moreover, the mean free paths are less than 9 nm, indicating that in the presence of grain boundaries the mobilities should not be effectively affected if the grain boundary size is tens of nanometers or larger. The ab initio approach demonstrated in this paper can be directly applied to other materials without the need for any a priori knowledge about the electron-phonon scattering processes, and can be straightforwardly extended to study cases with
Electrical resistivity of NaPb compound-forming liquid alloy using ab initio pseudopotentials
Anil Thakur; N S Negi; P K Ahluwalla
2005-08-01
The study of electrical resistivity of compound-forming liquid alloy, NaPb, is presented as a function of concentration. Hard sphere diameters of Na and Pb are obtained through the interionic pair potentials evaluated using Troullier and Martins ab initio pseudopotential, which have been used to calculate the partial structure factors (). Considering the liquid alloy to be a ternary mixture, Ziman formula, modified for complex formation has been used for calculating resistivity of binary liquid alloys. Form factors are calculated using ab initio pseudopotentials. The results suggest that Ziman formalism, when used with ab initio pseudopotentials, are quite successful in explaining the electrical resistivity data of compound-forming binary liquid alloys.
Londero, Elisa; Thiering, Gergő; Bijeikytė, Monika; Maze, Jeromino R.; Alkauskas, Audrius; Gali, Adam
2016-01-01
Understanding optical excitation spectra of point defects is still a scientific challenge. We demonstrate by \\emph{ab initio} calculations that a prominent sharp feature in the photoluminescence (PL) spectrum of the negatively charged silicon-vacancy defect in diamond can be only explained within the Herzberg-Teller approximation that goes beyond the commonly applied Franck-Condon approximation. The effect of the dynamic Jahn-Teller effect on the PL spectrum is also discussed. Our implementat...
Ab Initio Nuclear Structure Theory: From Few to Many
We summarize recent advances in ab initio nuclear structure theory, aiming to connect few- and many-body systems in a coherent theoretical framework. Starting from chiral effective field theory to construct the nuclear Hamiltonian and the similarity renormalization group to soften it, we address several many-body approaches that have seen major developments over the past few years. We show that the domain of ab initio nuclear structure theory has been pushed well beyond the p-shell and that quantitative predictions connected to QCD via chiral effective field theory are becoming possible all the way from the proton to the neutron drip line up into the medium-mass regime. (author)
Understanding phonon transport in thermoelectric materials using ab initio approaches
Broido, David
Good thermoelectric materials have low phonon thermal conductivity, kph. Accurate theories to describe kph are important components in developing predictive models of thermoelectric efficiency that can help guide synthesis and measurement efforts. We have developed ab initio approaches to calculate kph, in which phonon modes and phonon scattering rates are computed using interatomic force constants determined from density functional theory, and a full solution of the Boltzmann transport equation for phonons is implemented. A recent approach to calculate interatomic force constants using ab initio molecular dynamics has yielded a good description of the thermal properties of Bi2Te3. But, the complexity of new promising candidate thermoelectric materials introduces computational challenges in assessing their thermal properties. An example is germanane, a germanium based hydrogen-terminated layered semiconductor, which we will discuss in this talk.
The density matrix renormalization group for ab initio quantum chemistry
Wouters, Sebastian
2014-01-01
During the past 15 years, the density matrix renormalization group (DMRG) has become increasingly important for ab initio quantum chemistry. Its underlying wavefunction ansatz, the matrix product state (MPS), is a low-rank decomposition of the full configuration interaction tensor. The virtual dimension of the MPS, the rank of the decomposition, controls the size of the corner of the many-body Hilbert space that can be reached with the ansatz. This parameter can be systematically increased until numerical convergence is reached. The MPS ansatz naturally captures exponentially decaying correlation functions. Therefore DMRG works extremely well for noncritical one-dimensional systems. The active orbital spaces in quantum chemistry are however often far from one-dimensional, and relatively large virtual dimensions are required to use DMRG for ab initio quantum chemistry (QC-DMRG). The QC-DMRG algorithm, its computational cost, and its properties are discussed. Two important aspects to reduce the computational co...
We consider the question of the amorphization of metallic alloys by melt quenching, as predicted by molecular dynamics simulations with semi-empirical potentials. The parametrization of the potentials is discussed on the example of the ternary Cu-Ti-Zr transition metals alloy, using the ab-initio simulation as a reference. The pair structure in the amorphous state is computed from a potential of the Stillinger-Weber form. The transferability of the parameters during the quench is investigated using two parametrizations: from solid state data, as usual and from a new parametrization on the liquid structure. When the adjustment is made on the pair structure of the liquid, a satisfactory transferability is found between the pure components and their alloys. The liquid structure predicted in this way agrees well with experiment, in contrast with the one obtained using the adjustment on the solid. The final structure, after quenches down to the amorphous state, determined with the new set of parameters is shown to be very close to the ab-initio one, the latter being in excellent agreement with recent X-rays diffraction experiments. The corresponding critical temperature of the glass transition is estimated from the behavior of the heat capacity. Discussion on the consistency between the structures predicted using semi-empirical potentials and ab-initio simulation, and comparison of different experimental data underlines the question of the dependence of the final structure on the thermodynamic path followed to reach the amorphous state
Molexpl: a tool for ab initio data exploration and visualization
Wang, Xueying; Onofrio, Nicolas,; Strachan, Alejandro
2015-01-01
Density functional theory (DFT) based on ab initio theory, is a powerful method to resolve the electronic structure of atoms, molecules and solids. However, in practical, DFT is limited to few hundreds of atoms. To overcome this limitation, researchers have developed empirical interatomic potentials implemented in molecular dynamics (MD) simulations. MD ignores the movements of electrons and describes bonding and non-bonding interaction as a function of the distance between atoms called force...
Ab initio molecular dynamics simulation of laser melting of silicon
Silvestrelli, P.-L.; Alavi, A; Parrinello, M.; Frenkel, D
1996-01-01
The method of ab initio molecular dynamics, based on finite temperature density functional theory, is used to simulate laser heating of crystal silicon. We have found that a high concentration of excited electrons dramatically weakens the covalent bond. As a result, the system undergoes a melting transition to a metallic state. In contrast to ordinary liquid silicon, the new liquid is characterized by a high coordination number and a strong reduction of covalent bonding effects.
Towards an ab initio description of magnetism in ionic solids
Illas, F.; Casanovas, J.; García-Bach, M. A.; Caballol, R.; Castell, O.
1993-11-01
The physical contributions to the KNiF3 magnetic exchange coupling integral have been obtained from specially designed ab initio cluster model calculations. Three important mechanisms have been identified. These are the delocalization of the magnetic orbitals into the anion ``p'' band, the variational contribution of the second-order interactions, and the many-body terms ``hidden'' in the two-body operator and the Heisenberg Hamiltonian.
The density matrix renormalization group for ab initio quantum chemistry
Wouters, Sebastian
2015-01-01
During the past 15 years, the density matrix renormalization group (DMRG) has become increasingly important for ab initio quantum chemistry. It is used as a numerically exact solver for highly correlated regions in molecules. While the method works extremely well for one-dimensional systems, the correlated regions of interest are often far from one-dimensional. In this introductory talk, I will discuss the DMRG algorithm from a quantum information perspective, how quantum information theory h...
P-V Relation for Mercuric Calcogenides: Ab Initio Method
G. Misra; S. Tenguria; Gautam, M.
2011-01-01
Mercuric Calcogenides found many applications in electronic and optical devices as semiconducting materials. An equation of state provides useful information about the relationship between pressure (P), volume (V) and temperature (T) that helps to understand the behaviour of materials under the effect of high pressure and high temperature. The present paper sheds light on the electronic structure of Mercuric Calcogenides by simulating its electronic properties through ab initio method. This a...
Structure models: from shell model to ab initio methods
Bacca, Sonia
2016-01-01
A brief review of models to describe nuclear structure and reactions properties is presented, starting from the historical shell model picture and encompassing modern ab initio approaches. A selection of recent theoretical results on observables for exotic light and medium-mass nuclei is shown. Emphasis is given to the comparison with experiment and to what can be learned about three-body forces and continuum properties.
Ab-initio calculations for dilute magnetic semiconductors
Belhadji, Brahim
2008-01-01
This thesis focusses on ab-initio calculations for the electronic structure and the magnetic properties of dilute magnetic semiconductors (DMS). In particular we aim at the understanding of the complex exchange interactions in these systems. Our calculations are based on density functional theory, being ideally suited for a description of the material specific properties of the considered DMS. Moreover we use the KKR Green function method in connection with the coherent potential approximatio...
GAUSSIAN 76: An ab initio Molecular Orbital Program
Binkley, J. S.; Whiteside, R.; Hariharan, P. C.; Seeger, R.; Hehre, W. J.; Lathan, W. A.; Newton, M. D.; Ditchfield, R.; Pople, J. A.
1978-01-01
Gaussian 76 is a general-purpose computer program for ab initio Hartree-Fock molecular orbital calculations. It can handle basis sets involving s, p and d-type Gaussian functions. Certain standard sets (STO-3G, 4-31G, 6-31G*, etc.) are stored internally for easy use. Closed shell (RHF) or unrestricted open shell (UHF) wave functions can be obtained. Facilities are provided for geometry optimization to potential minima and for limited potential surface scans.
GAUSSIAN 76: an ab initio molecular orbital program
Binkley, J. S.; Whiteside, R.; Hariharan, P. C.; Seeger, R.; Hehre, W. J.; Lathan, W. A.; Newton, M. D.; Ditchfield, R.; Pople, J. A.
1978-06-01
Gaussian 76 is a general-purpose computer program for ab initio Hartree-Fock molecular orbital calculations. It can handle basis sets involving s, p and d-type gaussian functions. Certain standard sets (STO-3G, 4-31G, 6-31G*, etc.) are stored internally for easy use. Closed shell (RHF) or unrestricted open shell (UHF) wave functions can be obtained. Facilities are provided for geometry optimization to potential minima and for limited potential surface scans.
Der halbunendliche Kristall - Elektronische und optische Eigenschaften ab-initio
Brodersen, Sven
2002-01-01
Es werden die elektronischen und optischen Eigenschaften eines kristallinen Festkörpers unter Berücksichtigung der Oberfläche mit ab-initio Methoden berechnet. Die Behandlung der Oberflächeneffekte in einer Halbraum-Geometrie erzwingt die Darstellung der Wellenfunktionen und der Dielektrischen Funktion (DK) in einer lokalen Basis. Anhand von Volumenkristallen wird die Effizienz von LCAO- Basisfunktionen demonstriert. Die Erweiterung der atomaren Orbitale mit unbesetzten Orbitalen und 'off-sit...
Alnemrat, Sufian
Material science investigates the relationship between the structure of materials at the atomic or molecular scales and their macroscopic properties. Ab-initio DFT, atomistic force-field, and molecular dynamic simulations have been used to investigate the electronic, optical, structural, magnetic properties of group II-VI semiconductor nanoparticles, metal organic frameworks, amide-water complexes, and planetary core materials at the atomic and/or molecular level. Structure, density of electronic states, magnetic dipole moments, and HOMO-LUMO gaps of surface-passivated ZnnSem, Cd nTem, CdTe-core/ZnTe-shell, and ZnSe-core/CdSe-shell nanocrystals are calculated using a first principles. The intrinsic magnetic dipole moments are found to be strongly size dependent. The detailed analysis of the dipole moment as a function of particle size shows the appearance of zincblende-wurtzite polymorphism in these nano-particles. Energy-efficient adsorption processes are considered promising alternatives to traditional separation techniques. Mg-MOF-74, a magnesium-based metal organic framework, has been used as an efficient adsorbent structure for several gas separation purposes. Adsorption equilibria and kinetics of ethane, ethylene, propane, and propylene on Mg-MOF-74 were determined at temperatures of 278, 298, and 318 K and pressures up to 100 kPa. A grand canonical Monte Carlo simulation was conducted to calculate the adsorption isotherms and to explore adsorption mechanisms. I found that propylene and propane have a stronger affinity to the Mg-MOF-74 adsorbent than ethane and ethylene because of their significant dipole moments. Ab-initio molecular dynamics simulations were carried out to study the role of equilibrium volume and magnetism in Fe and FeX alloys (X=Ni, O) and their stability at earth core conditions. This study provides new insights into the pressure dependence of magnetism by tracking the hybridization between crystal orbitals for pressures up to 600 GPa in
Ab initio calculations for industrial materials engineering: successes and challenges
Wimmer, Erich; Freeman, Clive; Christensen, Mikael; Wolf, Walter; Saxe, Paul [Materials Design, Inc., PO Box 2000, Angel Fire, NM 87710 (United States); Najafabadi, Reza; Young Jr, George A; Ballard, Jake D; Angeliu, Thomas M; Vollmer, James [Knolls Atomic Power Laboratory, PO Box 1072, Schenectady, NY 12301-1072 (United States); Chambers, James J; Niimi, Hiroaki; Shaw, Judy B, E-mail: ewimmer@materialsdesign.co [Advanced CMOS, Texas Instruments Incorporated, Dallas, TX 75243 (United States)
2010-09-29
Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO{sub 2} junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.
Ab initio calculations for industrial materials engineering: successes and challenges
Computational materials science based on ab initio calculations has become an important partner to experiment. This is demonstrated here for the effect of impurities and alloying elements on the strength of a Zr twist grain boundary, the dissociative adsorption and diffusion of iodine on a zirconium surface, the diffusion of oxygen atoms in a Ni twist grain boundary and in bulk Ni, and the dependence of the work function of a TiN-HfO2 junction on the replacement of N by O atoms. In all of these cases, computations provide atomic-scale understanding as well as quantitative materials property data of value to industrial research and development. There are two key challenges in applying ab initio calculations, namely a higher accuracy in the electronic energy and the efficient exploration of large parts of the configurational space. While progress in these areas is fueled by advances in computer hardware, innovative theoretical concepts combined with systematic large-scale computations will be needed to realize the full potential of ab initio calculations for industrial applications.
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as “cloning,” in analogy to the “spawning” procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, “trains,” as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions
Ab initio calculations of reactions with light nuclei
Quaglioni, Sofia; Hupin, Guillaume; Calci, Angelo; Navrátil, Petr; Roth, Robert
2016-03-01
An ab initio (i.e., from first principles) theoretical framework capable of providing a unified description of the structure and low-energy reaction properties of light nuclei is desirable to further our understanding of the fundamental interactions among nucleons, and provide accurate predictions of crucial reaction rates for nuclear astrophysics, fusion-energy research, and other applications. In this contribution we review ab initio calculations for nucleon and deuterium scattering on light nuclei starting from chiral two- and three-body Hamiltonians, obtained within the framework of the ab initio no-core shell model with continuum. This is a unified approach to nuclear bound and scattering states, in which square-integrable energy eigenstates of the A-nucleon system are coupled to (A-a)+a target-plus-projectile wave functions in the spirit of the resonating group method to obtain an efficient description of the many-body nuclear dynamics both at short and medium distances and at long ranges.
A Complete and Accurate Ab Initio Repeat Finding Algorithm.
Lian, Shuaibin; Chen, Xinwu; Wang, Peng; Zhang, Xiaoli; Dai, Xianhua
2016-03-01
It has become clear that repetitive sequences have played multiple roles in eukaryotic genome evolution including increasing genetic diversity through mutation, changes in gene expression and facilitating generation of novel genes. However, identification of repetitive elements can be difficult in the ab initio manner. Currently, some classical ab initio tools of finding repeats have already presented and compared. The completeness and accuracy of detecting repeats of them are little pool. To this end, we proposed a new ab initio repeat finding tool, named HashRepeatFinder, which is based on hash index and word counting. Furthermore, we assessed the performances of HashRepeatFinder with other two famous tools, such as RepeatScout and Repeatfinder, in human genome data hg19. The results indicated the following three conclusions: (1) The completeness of HashRepeatFinder is the best one among these three compared tools in almost all chromosomes, especially in chr9 (8 times of RepeatScout, 10 times of Repeatfinder); (2) in terms of detecting large repeats, HashRepeatFinder also performed best in all chromosomes, especially in chr3 (24 times of RepeatScout and 250 times of Repeatfinder) and chr19 (12 times of RepeatScout and 60 times of Repeatfinder); (3) in terms of accuracy, HashRepeatFinder can merge the abundant repeats with high accuracy. PMID:26272474
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics
Makhov, Dmitry V.; Glover, William J.; Martinez, Todd J.; Shalashilin, Dmitrii V.
2014-08-01
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as "cloning," in analogy to the "spawning" procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, "trains," as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions.
Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics
Makhov, Dmitry V.; Shalashilin, Dmitrii V. [Department of Chemistry, University of Leeds, Leeds LS2 9JT (United Kingdom); Glover, William J.; Martinez, Todd J. [Department of Chemistry and The PULSE Institute, Stanford University, Stanford, California 94305, USA and SLAC National Accelerator Laboratory, Menlo Park, California 94025 (United States)
2014-08-07
We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as “cloning,” in analogy to the “spawning” procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, “trains,” as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions.
Ab initio nuclear structure - the large sparse matrix eigenvalue problem
The structure and reactions of light nuclei represent fundamental and formidable challenges for microscopic theory based on realistic strong interaction potentials. Several ab initio methods have now emerged that provide nearly exact solutions for some nuclear properties. The ab initio no core shell model (NCSM) and the no core full configuration (NCFC) method, frame this quantum many-particle problem as a large sparse matrix eigenvalue problem where one evaluates the Hamiltonian matrix in a basis space consisting of many-fermion Slater determinants and then solves for a set of the lowest eigenvalues and their associated eigenvectors. The resulting eigenvectors are employed to evaluate a set of experimental quantities to test the underlying potential. For fundamental problems of interest, the matrix dimension often exceeds 1010 and the number of nonzero matrix elements may saturate available storage on present-day leadership class facilities. We survey recent results and advances in solving this large sparse matrix eigenvalue problem. We also outline the challenges that lie ahead for achieving further breakthroughs in fundamental nuclear theory using these ab initio approaches.
Clouthier, Dennis J., E-mail: dclaser@uky.edu [Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055 (United States)
2014-12-28
The F{sub 2}BO free radical is a known, although little studied, species but similar X{sub 2}BY (X = H, D, F; Y = O, S) molecules are largely unknown. High level ab initio methods have been used to predict the molecular structures, vibrational frequencies (in cm{sup −1}), and relative energies of the ground and first two excited electronic states of these free radicals, as an aid to their eventual spectroscopic identification. The chosen theoretical methods and basis sets were tested on F{sub 2}BO and found to give good agreement with the known experimental quantities. In particular, complete basis set extrapolations of coupled-cluster single and doubles with perturbative triple excitations/aug-cc-pVXZ (X = 3, 4, 5) energies gave excellent electronic term values, due to small changes in geometry between states and the lack of significant multireference character in the wavefunctions. The radicals are found to have planar C{sub 2v} geometries in the X{sup ~2}B{sub 2} ground state, the low-lying A{sup ~2}B{sub 1} first excited state, and the higher B{sup ~2}A{sub 1} state. Some of these radicals have very small ground state dipole moments hindering microwave measurements. Infrared studies in matrices or in the gas phase may be possible although the fundamentals of H{sub 2}BO and H{sub 2}BS are quite weak. The most promising method of identifying these species in the gas phase appears to be absorption or laser-induced fluorescence spectroscopy through the allowed B{sup ~}-X{sup ~} transitions which occur in the visible-near UV region of the electromagnetic spectrum. The ab initio results have been used to calculate the Franck-Condon profiles of the absorption and emission spectra, and the rotational structure of the B{sup ~}-X{sup ~}0{sub 0}{sup 0} bands has been simulated. The calculated single vibronic level emission spectra provide a unique, readily recognizable fingerprint of each particular radical, facilitating the experimental identification of new X{sub 2}BY
Bernard, St
1998-12-31
The quest for metallic hydrogen is a major goal for both theoretical and experimental condensed matter physics. Hydrogen and deuterium have been compressed up to 200 GPa in diamond anvil cells, without any clear evidence for a metallic behaviour. Loubeyere has recently suggested that hydrogen could metallize, at pressures within experimental range, in a new Van der Waals compound: Ar(H{sub 2}){sub 2} which is characterized at ambient pressure by an open and anisotropic sublattice of hydrogen molecules, stabilized by an argon skeleton. This thesis deals with a detailed ab initio investigation, by Car-Parrinello molecular dynamics methods, of the evolution under pressure of this compound. In a last chapter, we go to much higher pressures and temperatures, in order to compare orbital and orbital free ab initio methods for the dense hydrogen plasma. (author) 109 refs.
Olsson, Emilia; Aparicio-Anglès, Xavier; de Leeuw, Nora H.
2016-07-01
Doped LaMnO3 and SmCoO3 are important solid oxide fuel cell cathode materials. The main difference between these two perovskites is that SmCoO3 has proven to be a more efficient cathode material than LaMnO3 at lower temperatures. In order to explain the difference in efficiency, we need to gain insight into the materials' properties at the atomic level. However, while LaMnO3 has been widely studied, ab initio studies on SmCoO3 are rare. Hence, in this paper, we perform a comparative DFT + U study of the structural, electronic, and magnetic properties of these two perovskites. To that end, we first determined a suitable Hubbard parameter for the Co d-electrons to obtain a proper description of SmCoO3 that fully agrees with the available experimental data. We next evaluated the impact of oxygen and cation vacancies on the geometry, electronic, and magnetic properties. Oxygen vacancies strongly alter the electronic and magnetic structures of SmCoO3, but barely affect LaMnO3. However, due to their high formation energy, their concentrations in the material are very low and need to be induced by doping. Studying the cation vacancy concentration showed that the formation of cation vacancies is less energetically favorable than oxygen vacancies and would thus not markedly influence the performance of the cathode.
Grimminger, Robert; Clouthier, Dennis J., E-mail: dclaser@uky.edu [Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055 (United States); Sheridan, Phillip M. [Department of Chemistry and Biochemistry, Canisius College, Buffalo, New York 14208 (United States)
2014-04-28
We have studied the B{sup ~} {sup 2}A{sub 1}–X{sup ~} {sup 2}B{sub 2} laser-induced fluorescence (LIF) spectrum of the jet-cooled F{sub 2}BO radical for the first time. The transition consists of a strong 0{sub 0}{sup 0} band at 446.5 nm and eight weak sequence bands to shorter wavelengths. Single vibronic level emission spectra obtained by laser excitation of individual levels of the B{sup ~} state exhibit two electronic transitions: a very weak, sparse B{sup ~}–X{sup ~} band system in the 450–500 nm region and a stronger, more extensive set of B{sup ~} {sup 2}A{sub 1}–A{sup ~} {sup 2}B{sub 1} bands in the 580–650 nm region. We have also performed a series of high level ab initio calculations to predict the electronic energies, molecular structures, vibrational frequencies, and rotational and spin-rotation constants in the X{sup ~} {sup 2}B{sub 2}, A{sup ~2}B{sub 1} and B{sup ~} {sup 2}A{sub 1} electronic states as an aid to the analysis of the experimental data. The theoretical results have been used as input for simulations of the rotationally resolved B{sup ~} {sup 2}A{sub 1}–X{sup ~} {sup 2}B{sub 2} 0{sub 0}{sup 0} LIF band and Franck-Condon profiles of the LIF and single vibronic level emission spectra. The agreement between the simulations obtained with purely ab initio parameters and the experimental spectra validates the geometries calculated for the ground and excited states and the conclusion that the radical has C{sub 2v} symmetry in the X{sup ~}, A{sup ~}, and B{sup ~} states. The spectra provide considerable new information about the vibrational energy levels of the X{sup ~} and A{sup ~} states, but very little for the B{sup ~} state, due to the very restrictive Franck-Condon factors in the LIF spectra.
Experimental and ab initio studies on sub-lattice ordering and magnetism in Co2Fe(Ge1−xSix) alloys
Crystallographic and magnetic properties of bulk Co2Fe(Ge1−xSix) alloys with 0 ≤ x ≤ 1, synthesized by arc melting method, have been studied. Co2FeSi alloy has been found to crystallize with L21 structure, but the super-lattice peaks are absent in the X-ray diffraction patterns of alloys containing high Ge concentration. Unit cell volume of this series of alloys decreased from 185.2 to 178.5 Å3 as Si content was increased from 0 to 1.00. All alloy compositions exhibit ferromagnetic behavior with a high Curie temperature (TC). TC showed a systematic variation with x. A comparison between the values of saturation magnetization (Ms) and effective moment per magnetic atom pc estimated from the temperature dependent susceptibility data above TC, shows that the alloys have half-metallic character. The alloy with x = 0 follows Slater-Pauling (S-P) rule with Ms of 5.99μB. However, Ms for the alloy with x = 1.00 was found to be 5.42μB, which is lower than the value of 6.0μB predicted by S-P rule. Since atomic disorder is known to affect the Ms and electronic structure of these alloys, ab initio calculations were carried out to explain the deviation in observed Ms from S-P rule prediction and the half-metallic character of the alloys. Ab initio calculations reveal that alloys with L21 structure have Ms value as predicted by S-P rule. However, introduction of 12.5% DO3 disorder, which occurs due to swapping of Co and Fe atoms in the unit cell, decreases Ms of alloys with x > 0 from the S-P prediction to values obtained experimentally. The results analyzed from the view point of electronic structure of the alloys in different ordered states bring out the influence of disorder on the observed magnetic properties of these technologically important alloys
Ab initio Defect Energetics in LaBO3 Perovskite Solid Oxide Fuel Cell Materials
Lee, Yueh-Lin; Morgan, Dane; Kleis, Jesper;
2009-01-01
Perovskite materials of the form ABO3 are a promising family of compounds for use in solid oxide fuel cell (SOFC) cathodes. Study of the physics of these compounds under SOFC conditions with ab initio methods is particularly challenging due to high temperatures, exchange of oxygen with O2 gas, and...
Ab initio molecular dynamics simulation of hydrogen fluoride at several thermodynamic states
Kreitmeir, M.; Bertagnolli, H.; Mortensen, Jens Jørgen;
2003-01-01
Liquid hydrogen fluoride is a simple but interesting system for studies of the influence of hydrogen bonds on physical properties. We have performed ab initio molecular dynamics simulations of HF at several thermodynamic states, where we examine the microscopic structure of the liquid as well as...
Ab initio molecular crystal structures, spectra, and phase diagrams.
Hirata, So; Gilliard, Kandis; He, Xiao; Li, Jinjin; Sode, Olaseni
2014-09-16
Conspectus Molecular crystals are chemists' solids in the sense that their structures and properties can be understood in terms of those of the constituent molecules merely perturbed by a crystalline environment. They form a large and important class of solids including ices of atmospheric species, drugs, explosives, and even some organic optoelectronic materials and supramolecular assemblies. Recently, surprisingly simple yet extremely efficient, versatile, easily implemented, and systematically accurate electronic structure methods for molecular crystals have been developed. The methods, collectively referred to as the embedded-fragment scheme, divide a crystal into monomers and overlapping dimers and apply modern molecular electronic structure methods and software to these fragments of the crystal that are embedded in a self-consistently determined crystalline electrostatic field. They enable facile applications of accurate but otherwise prohibitively expensive ab initio molecular orbital theories such as Møller-Plesset perturbation and coupled-cluster theories to a broad range of properties of solids such as internal energies, enthalpies, structures, equation of state, phonon dispersion curves and density of states, infrared and Raman spectra (including band intensities and sometimes anharmonic effects), inelastic neutron scattering spectra, heat capacities, Gibbs energies, and phase diagrams, while accounting for many-body electrostatic (namely, induction or polarization) effects as well as two-body exchange and dispersion interactions from first principles. They can fundamentally alter the role of computing in the studies of molecular crystals in the same way ab initio molecular orbital theories have transformed research practices in gas-phase physical chemistry and synthetic chemistry in the last half century. In this Account, after a brief summary of formalisms and algorithms, we discuss applications of these methods performed in our group as compelling
Suendo, Veinardi
2011-01-01
Chlorophyll a is one the most abundant pigment on Earth, which is responsible for trapping the light energy to perform the photosynthesis process in green plants. This molecule is a metal-complex compound that consists of a porphyrins ring with high symmetry that acts as ligands with magnesium as the central ion. Chlorophyll a has been studied for many years from different point of views for both experimental and theoretical interests. In this study, the restricted Hartree-Fock configuration interaction single (RHF/CIS), time-dependent density functional theory (TDDFT) and some semi-empirical methods (CNDO/s and ZINDO) calculations were carried out and compared to reconstruct the UV-Vis absorption spectra of chlorophyll a. In some extend, the calculation results based on a single molecule calculation were succeeded to reconstruct the absorption spectra but required to be scaling and broaden to match the experimental one. Different computational methods (ab initio and semi-empirical) exhibits the differences i...
Song, Hongwei; Lu, Yunpeng; Li, Jun; Yang, Minghui; Guo, Hua
2016-04-28
An initial state selected time-dependent wave packet method is applied to study the dynamics of the OH + CHD3 reaction with a six-dimensional model on a newly developed full-dimensional ab initio potential energy surface (PES). This quantum dynamical (QD) study is complemented by full-dimensional quasi-classical trajectory (QCT) calculations on the same PES. The QD results indicate that both translational energy and the excitation of the CH stretching mode significantly promote the reaction while the excitation of the umbrella mode has a negligible effect on the reactivity. For this early barrier reaction, interestingly, the CH stretching mode is more effective than translational energy in promoting the reaction except at very low collision energies. These QD observations are supported by QCT results. The higher efficacy of the CH stretching model in promoting this early barrier reaction is inconsistent with the prediction of the naively extended Polanyi's rules, but can be rationalized by the recently proposed sudden vector projection model. PMID:27131546
Veinardi Suendo
2012-07-01
Full Text Available Chlorophyll a is one the most abundant pigment on Earth that responsible for trapping the light energy to perform photosynthesis in green plants. This molecule has been studied for many years from different point of views in both experimental and theoretical interests. In this study, the restricted Hartree-Fock configuration interaction single (RHF/CIS, time-dependent density functional theory (TDDFT and several semi-empirical methods (CNDO/s and ZINDO calculations were carried out to reconstruct the UV-Vis absorption spectra of chlorophyll a. In some extend, the calculation results based on single molecule approach were succeeded to reconstruct the absorption spectra but required to be rescaled to fit the experimental one. In general, the semi-empirical methods provide better energy scaling factor that closer to unity. However, they lack of vertical transition fine features with respect to the spectrum obtained experimentally. Here, the ab initio calculations provide more complete features, especially the TDDFT at high level of basis sets that also has a good accuracy in the transition energies. The contribution of ground states and excited states orbitals in the main vertical transitions is discussed based on delocalization nature of the wavefunctions and the presence of solvent through polarizable continuum model (PCM.
Atomic structure, electronic structure and properties of ZrBCN materials were studied using ab-initio calculations. We focus on the effect of N and Zr content and compare our results with the characteristics of samples prepared by reactive magnetron sputtering. We find that the materials are characterized by a mixture of covalent and metallic bonding. The metallic-to-covalent ratio is primarily controlled by the N content, and well correlated with measured electrical resistivity. Zr-N bonds have the highest preference to form out of all bonds in the ZrBCN system. Hard nanostructure forms at high Zr and low N content, containing (i) pure ZrN and (ii) (theoretically predicted and experimentally observed) solid solution close to ZrB0.50N0.50. The results allow one to tailor ZrBCN compositions which can combine different functional properties, such as high thermal stability, high hardness and electrical conductivity. - Highlights: • Resistivity of ZrBCN films of various compositions spans from 10−6 to > 107 Ωm. • Calculations show a mixture of metallic and covalent bonding controlled by [N]. • Zr-N bonds have the highest preference to form out of all bonds in the ZrBCN system. • Calculations explain the presence of two crystalline phases at high [Zr] and low [N]. • Results allow to tailor ZrBCN compositions combining different functional properties
Fischer, Sean A.; Ueltschi, Tyler W.; El-Khoury, Patrick Z.; Mifflin, Amanda L.; Hess, Wayne P.; Wang, Hongfei; Cramer, Christopher J.; Govind, Niranjan
2016-03-03
Carbon-hydrogen (C-H) vibration modes serve as key probes in the chemical iden- tication of hydrocarbons and in vibrational sum-frequency generation (SFG) spec- *troscopy of hydrocarbons at the liquid/gas interface. Their assignments pose a chal- lenge from a theoretical viewpoint. In this work, we present a detailed study of the C-H stretching region of dimethyl sulfoxide (DMSO) using a new Gaussian basis set- based ab initio molecular dynamics (AIMD) module that we have implemented in the NWChem computational chemistry program. By combining AIMD simulations and static normal mode analysis, we interpret experimental infrared and Raman spectra and explore the role of anharmonic effects in this system. Our anharmonic normal mode analysis of the in-phase and out-of-phase symmetric C-H stretching modes chal- lenges the previous experimental assignment of the shoulder in the symmetric C-H stretching peak as an overtone or Fermi resonance. In addition, our AIMD simulations also show signicant broadening of the in-phase symmetric C-H stretching resonance, which suggests that the experimentally observed shoulder is due to thermal broadening of the symmetric stretching resonance.
Ab initio and semiempirical calculations of large cluster models have been performed in order to study water adsorption and dissociation on pure, defective (vacancies) and doped (Li, Na, K, Ca, Fe) MgO (001) surfaces. The geometries of the adsorbed and dissociated molecules have been optimized preparatory to analysis of binding energies, stretching frequencies, charge transfers, preferential sites of interaction, and bond distances. We have used Mulliken, natural bond order, and electrostatic-derived atomic and overlap populations to analyze charge distributions in the clusters. We have also investigated transition structures, activation energies, energy gaps, HOMO, density of states, SCF orbital energies as well as the acid endash base properties of our cluster model. Numerical results are compared, where possible, with experiment, interpreted in the framework of various analytical models, and correlated with site coordination numbers, corner and edge site preferential locations, and direction of charge transfer. A thorough charge analysis indicates substantial charge redistribution in the magnesium oxide crystal as a result of water adsorption and dissociation in pure, defective, and doped MgO crystals. The introduction of heavier impurities and vacancies could produce substantial changes in the physical and chemical properties of the catalyst and increase the binding and dissociation energies. Some of the largest changes originate from the introduction of vacancies. Two and three-dimensional potential energy surfaces are used to investigate activation energies of hydroxylation on the MgO surface. Stretching frequencies are correlated with magnesium and oxygen coordination numbers. copyright 1998 American Institute of Physics
Ab-initio study of the interfacial properties in ultrathin MgO films on O-rich FeO/Fe(001) surfaces
Jeon, Junjin; Yu, Byungdeok [University of Seoul, Seoul (Korea, Republic of)
2014-09-15
Using ab-initio simulations based on density functional theory, we systematically studied the interfacial properties of MgO films on O-rich FeO/Fe(001) surfaces with increasing number of MgO layers from one to three monolayers (MLs). The structural and the adhesion properties of the MgO/FeO/Fe(001) system were assessed and compared with those of simple MgO/Fe(001) interfaces. Our calculated results showed that the adhesion energy for MgO/FeO/Fe(001) was smaller than that for simple MgO/Fe(001). An analysis of the electronic structures and the charge rearrangements of the MgO/FeO/Fe(001) interfaces was also performed. The work functions of the MgO/FeO/Fe(001) systems upon the deposition of MgO films exhibited smaller decreases (0.45 - 0.67 eV) than those (1.43 - 1.74 eV) of the MgO/Fe(001) systems. In addition, the obtained work functions (3.77 - 3.99 eV) for MgO/FeO/Fe(001) were much larger than those (2.12 - 2.43 eV) for MgO/Fe(001).
The first theoretical exploration of superhalogen properties of polynuclear structures based on pseudohalogen ligand is reported here via a case study on eight triply-bridged [Mg2(CN)5]− clusters. From our high-level ab initio results, all these clusters are superhalogens due to their high vertical electron detachment energies (VDE), of which the largest value is 8.67 eV at coupled-cluster single double triple (CCSD(T)) level. Although outer valence Green's function results are consistent with CCSD(T) in most cases, it overestimates the VDEs of three anions dramatically by more than 1 eV. Therefore, the combined usage of several theoretical methods is important for the accuracy of purely theoretical prediction of superhalogen properties of new structures. Spatial distribution of the extra electron of high-VDE anions here indicates two features: remarkable aggregation on bridging CN units and non-negligible distribution on every CN unit. These two features lower the potential and kinetic energies of the extra electron respectively and thus lead to high VDE. Besides superhalogen properties, the structures, relative stabilities and thermodynamic stabilities with respect to detachment of CN−1 were also investigated for these anions. The collection of these results indicates that polynuclear structures based on pseudohalogen ligand are promising candidates for new superhalogens with enhanced properties
The static and dynamic properties of liquid Fe at high pressure and temperature have been studied using an ab initio molecular dynamics method. We have focused on four thermodynamic states at pressures of 27, 42, 50 and 58 GPa for which x-ray scattering data are available. The calculated static structure shows very good agreement with the available experimental data, including an asymmetric second peak which becomes more marked with increasing pressure. The dynamical structure reveals the existence of propagating density fluctuations and the associated dispersion relation has also been determined. The relaxation mechanisms for the density fluctuations have been analyzed in terms of a model with two decay channels (fast and slow, respectively). We found that the thermal relaxation proceeds along the slow decaying channel whereas the fast one is that of the viscoelastic relaxation. The possible coupling between longitudinal and transverse excitation modes has been investigated by looking at specific signatures in two wavevector regions: the first one is located around the position of the main peak of the structure factor, q p, as suggested by the recently reported appearance of high frequency transverse waves in liquid Li under high pressures; the second region is around q p/2, as suggested by the recent finding of transverse acoustic modes in inelastic x-ray scattering intensities of liquid Fe at ambient pressure. Finally, results are also reported for several transport coefficients. (paper)
Ilyasov, V.V., E-mail: viily@mail.ru; Pham, Khang D., E-mail: dinhkhang307@gmail.com; Holodova, O.M.; Ershov, I.V., E-mail: thijd@mail.ru
2015-10-01
We have performed ab initio simulation of oxygen atom adsorption on TiC(0 0 1) laser-reconstructed surface. Relaxed atomic structures of the O/Ti{sub x}C{sub y}(0 0 1) surface observed upon thermal impact have been studied. DFT calculations of their thermodynamic, electronic, and elastic properties have been carried out. For the first time we have established the bond length and adsorption energy for various reconstructions of the O/Ti{sub x}C{sub y}(0 0 1) surface atomic structure. We have examined the effects of the oxygen adatom upon the band and electron spectra of the O/TiC(0 0 1) surface in its various reconstructions. For the first time we have established a correlation between the energy level of flat bands (−5.4 eV and −5.8 eV) responsible for the doublet of singular peaks of partial densities of oxygen 2p electrons, and the adsorption energy of oxygen atom in non-stoichiometric O/TiC{sub y}(0 0 1) systems. Effective charges of titanium and carbon atoms surrounding the oxygen adatom in various reconstructions have been identified. We have established charge transfer from titanium atom to oxygen and carbon atoms determined by the reconstruction of local atomic and electron structures which correlate with atomic electronegativity values and chemisorption processes. Potential mechanisms for laser nanostructuring of titanium carbide surface have been suggested.
Tütüncü, H. M. [Sakarya Üniversitesi, Fen-Edebiyat Fakültesi, Fizik Bölümü, 54187 Adapazarı (Turkey); Sakarya Üniversitesi, Biyomedikal, Manyetik ve Yarıiletken Malzemeler Araştırma Merkezi (BIMAYAM), 54187 Adapazarı (Turkey); Srivastava, G. P. [School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL (United Kingdom)
2015-04-21
Ab initio studies of the electronic band structure and phonon dispersion relations, using the planewave pseudopotential method and the density functional theory, have been made for the superconducting materials YC{sub 2} and LaC{sub 2}. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges between these materials are investigated and discussed. By integrating the Eliashberg spectral function α{sup 2}F(ω), the average electron-phonon coupling parameter is found to be λ = 0.55 for YC{sub 2} and 0.54 for LaC{sub 2}, indicating these to be weak-coupling BCS superconductors. It is established that about 60% of λ is contributed by acoustic phonons in both materials. Using a reasonable value of μ{sup * }= 0.13 for the effective Coulomb repulsion parameter, the superconducting critical temperature T{sub c} is found to be 3.81 K for YC{sub 2} and 2.44 K for LaC{sub 2}, in good agreement with values reported from experimental measurements.
Timoshevskii, Vladimir; Feng, Zimin; Bevan, Kirk H; Zaghib, Karim
2015-08-26
The atomic and electronic structures of the LiFePO4 (LFP) surface, both bare and reconstructed upon possible oxygenation, are theoretically studied by ab initio methods. On the basis of total energy calculations, the atomic structure of the oxygenated surface is proposed, and the effect of surface reconstruction on the electronic properties of the surface is clarified. While bare LFP(010) surface is insulating, adsorption of oxygen leads to the emergence of semimetallic behavior by inducing the conducting states in the band gap of the system. The physical origin of these conducting states is investigated. We further demonstrate that deposition of Li2S layers on top of oxygenated LFP(010) surface leads to the formation of additional conducting hole states in the first layer of Li2S surface because of the charge transfer from sulfur p-states to the gap states of LFP surface. This demonstrates that oxygenated LFP surface not only provides conducting layers itself, but also induces conducting channels in the top layer of Li2S. These results help to achieve further understanding of potential role of LFP particles in improving the performance of Li-S batteries through emergent interface conductivity. PMID:26237114
Jinghan Zou; Shuhui Yin; Dan Wu; Mingxing Guo; Xuesong Xu; Hong Gao; Lei Li; Li Che
2013-09-01
Theoretical study on the dynamics of reactions H' + HS( = 0, = 0)→H2 + S and H' + HS( =0, = 0)→ H + H'S is performed with quasi-classical trajectory (QCT) method on a new ab initio potential energy surface for the lowest triplet state of H2S (3A") constructed in 2012 by Lv et al. The QCT-calculated reaction integral cross-sections are in good agreement with previous quantum wave packet results over the collision energy range of 0-50 kcal/mol. Both the abstraction and exchange reactions are governed by direct reaction dynamics and the trajectories follow the minimum energy path. The rotational angular momentum vector ' of products in the two reaction channels are not only aligned perpendicular to scattering plane but also oriented along the negative direction of the axis perpendicular to the scattering plane. With the increase in collision energy, the variation trends of product polarization in the two reaction channels are different and that may be attributed to the obviously different characteristic of the two channels on the potential energy surface.
Ab-initio study of the interfacial properties in ultrathin MgO films on O-rich FeO/Fe(001) surfaces
Using ab-initio simulations based on density functional theory, we systematically studied the interfacial properties of MgO films on O-rich FeO/Fe(001) surfaces with increasing number of MgO layers from one to three monolayers (MLs). The structural and the adhesion properties of the MgO/FeO/Fe(001) system were assessed and compared with those of simple MgO/Fe(001) interfaces. Our calculated results showed that the adhesion energy for MgO/FeO/Fe(001) was smaller than that for simple MgO/Fe(001). An analysis of the electronic structures and the charge rearrangements of the MgO/FeO/Fe(001) interfaces was also performed. The work functions of the MgO/FeO/Fe(001) systems upon the deposition of MgO films exhibited smaller decreases (0.45 - 0.67 eV) than those (1.43 - 1.74 eV) of the MgO/Fe(001) systems. In addition, the obtained work functions (3.77 - 3.99 eV) for MgO/FeO/Fe(001) were much larger than those (2.12 - 2.43 eV) for MgO/Fe(001).
Order–disorder phase transitions induced by thermal annealing have been studied in the ordered-vacancy compound ZnGa2Se4 by means of Raman scattering and optical absorption measurements. The partially disordered as-grown sample with tetragonal defect stannite (DS) structure and I 4-bar 2 m space group has been subjected to controlled heating and cooling cycles. In situ Raman scattering measurements carried out during the whole annealing cycle show that annealing the sample to 400 °C results in a cation ordering in the sample, leading to the crystallization of the ordered tetragonal defect chalcopyrite (DC) structure with I 4-bar space group. On decreasing temperature the ordered cation scheme of the DC phase can be retained at ambient conditions. The symmetry of the Raman-active modes in both DS and DC phases is discussed and the similarities and differences between the Raman spectra of the two phases emphasized. The ordered structure of annealed samples is confirmed by optical absorption measurements and ab initio calculations, that show that the direct bandgap of DC-ZnGa2Se4 is larger than that of DS-ZnGa2Se4. (paper)
Lee, T. H.; Simdyankin, S. I.; Hegedus, J.; Heo, J.; Elliott, S. R.
2010-03-01
The spatial distribution of Nd3+ ions and GaS4 tetrahedral units in Nd-doped Ge-As-Ga-S glasses has been studied by laser spectroscopy and ab initio molecular dynamics (MD) simulations. A sharp increase in Nd3+ fluorescence intensities and lifetimes was observed with increasing Ga content, and attributed to the formation of tightly bound Nd3+ clusters in Ga-free glasses and the subsequent dissolution of such clusters upon Ga doping. A large modification in Nd3+ sites was also identified from low-temperature site-selective excitation spectra, suggesting preferential spatial correlations between Nd3+ and GaS4 tetrahedra even at low Ga-doping levels. MD simulations of these materials in the liquid state showed a tendency for Ga cluster formation as well as spatial correlations between Nd and Ga atoms consistent with the experimental results. On the basis of this result, a comprehensive structural model for Nd- and Ga-doped sulfide glasses is proposed.
For our study we have chosen a series of diatomic molecules MeB (where Me = Li, Na, K, Rb, Cs, Fr). These molecules present experimentally unknown species, hence we were motivated to predict theoretically potential energy curves, equilibrium bond lengths, harmonic frequencies, constants of anharmonicity, dipole moments and dissociation energies for the ground and low-lying excited states using high level ab initio techniques. Based on previous state averaged MRCI calculations in ANO-S basis set of NaB and KB molecules, we have focused on four lowest-lying electronic states, ground state 3Π and excited states 1Σ+, 1Π and 3Σ+. All four states dissociate to the atoms in ground states 2P1/2(B) and 2S1/2(Me). 3Π, 1Σ+, 1Π and 3Σ+ electronic states we investigated employing CCSD(T) method using relativistic ANO-RCC basis set. Our calculations include scalar relativistic effects via the second order one-component (spin-free) Douglas-Kroll-Hess Hamiltonian. Relativistic effects become remarkable in the case of heavy atoms, hence properties of CsB and FrB molecules may differ from trend of properties in row from LiB to FrB. Spectroscopic properties of particular state were obtained from the analysis of the potential energy curves using VIBROT and DUNHAM programs.
Marqués, Miriam; González, Luis E.; González, David J.
2016-02-01
The static and dynamic properties of liquid Fe at high pressure and temperature have been studied using an ab initio molecular dynamics method. We have focused on four thermodynamic states at pressures of 27, 42, 50 and 58 GPa for which x-ray scattering data are available. The calculated static structure shows very good agreement with the available experimental data, including an asymmetric second peak which becomes more marked with increasing pressure. The dynamical structure reveals the existence of propagating density fluctuations and the associated dispersion relation has also been determined. The relaxation mechanisms for the density fluctuations have been analyzed in terms of a model with two decay channels (fast and slow, respectively). We found that the thermal relaxation proceeds along the slow decaying channel whereas the fast one is that of the viscoelastic relaxation. The possible coupling between longitudinal and transverse excitation modes has been investigated by looking at specific signatures in two wavevector regions: the first one is located around the position of the main peak of the structure factor, q p , as suggested by the recently reported appearance of high frequency transverse waves in liquid Li under high pressures; the second region is around q p /2, as suggested by the recent finding of transverse acoustic modes in inelastic x-ray scattering intensities of liquid Fe at ambient pressure. Finally, results are also reported for several transport coefficients.
Marqués, Miriam; González, Luis E; González, David J
2016-02-24
The static and dynamic properties of liquid Fe at high pressure and temperature have been studied using an ab initio molecular dynamics method. We have focused on four thermodynamic states at pressures of 27, 42, 50 and 58 GPa for which x-ray scattering data are available. The calculated static structure shows very good agreement with the available experimental data, including an asymmetric second peak which becomes more marked with increasing pressure. The dynamical structure reveals the existence of propagating density fluctuations and the associated dispersion relation has also been determined. The relaxation mechanisms for the density fluctuations have been analyzed in terms of a model with two decay channels (fast and slow, respectively). We found that the thermal relaxation proceeds along the slow decaying channel whereas the fast one is that of the viscoelastic relaxation. The possible coupling between longitudinal and transverse excitation modes has been investigated by looking at specific signatures in two wavevector regions: the first one is located around the position of the main peak of the structure factor, qp, as suggested by the recently reported appearance of high frequency transverse waves in liquid Li under high pressures; the second region is around qp/2, as suggested by the recent finding of transverse acoustic modes in inelastic x-ray scattering intensities of liquid Fe at ambient pressure. Finally, results are also reported for several transport coefficients. PMID:26811899
Ji, Min; Zhang, Qun; Chen, Yang
2009-01-01
We report on the photodissociation dynamics study of n-butyl nitrite (n-C_4H_9ONO) and isoamyl nitrite ((CH_3)_2C_3H_5ONO) by means of time-resolved Fourier transform infrared (TR-FTIR) emission spectroscopy. The obtained TR-FTIR emission spectra of the nascent NO fragments produced in the 355-nm laser photolysis of the two alkyl nitrite species showed an almost identical rotational temperature and vibrational distributions of NO. In addition, a close resemblance between the two species was also found in the measured temporal profiles of the IR emission of NO and the recorded UV absorption spectra. The experimental results are consistent with our ab initio calculations using the time-dependent density functional theory at the B3LYP/6-311G(d,p) level, which indicate that the substitution of one of the two {gamma}-H atoms in n-C_4H_9ONO with a methyl group to form (CH_3)_2C_3H_5ONO has only a minor effect on the photodissociation dynamics of the two molecules.
Chen, Liuyang; Shao, Kejie; Chen, Jun; Yang, Minghui; Zhang, Dong H
2016-05-21
This work performs a time-dependent wavepacket study of the H2 + C2H → H + C2H2 reaction on a new ab initio potential energy surface (PES). The PES is constructed using neural network method based on 68 478 geometries with energies calculated at UCCSD(T)-F12a/aug-cc-pVTZ level and covers H2 + C2H↔H + C2H2, H + C2H2 → HCCH2, and HCCH2 radial isomerization reaction regions. The reaction dynamics of H2 + C2H → H + C2H2 are investigated using full-dimensional quantum dynamics method. The initial-state selected reaction probabilities are calculated for reactants in eight vibrational states. The calculated results showed that the H2 vibrational excitation predominantly enhances the reactivity while the excitation of bending mode of C2H slightly inhibits the reaction. The excitations of two stretching modes of C2H molecule have negligible effect on the reactivity. The integral cross section is calculated with J-shift approximation and the mode selectivity in this reaction is discussed. The rate constants over 200-2000 K are calculated and agree well with the experimental measured values. PMID:27208951
Ab initio charge-carrier mobility model for amorphous molecular semiconductors
Massé, Andrea; Friederich, Pascal; Symalla, Franz; Liu, Feilong; Nitsche, Robert; Coehoorn, Reinder; Wenzel, Wolfgang; Bobbert, Peter A.
2016-05-01
Accurate charge-carrier mobility models of amorphous organic molecular semiconductors are essential to describe the electrical properties of devices based on these materials. The disordered nature of these semiconductors leads to percolative charge transport with a large characteristic length scale, posing a challenge to the development of such models from ab initio simulations. Here, we develop an ab initio mobility model using a four-step procedure. First, the amorphous morphology together with its energy disorder and intermolecular charge-transfer integrals are obtained from ab initio simulations in a small box. Next, the ab initio information is used to set up a stochastic model for the morphology and transfer integrals. This stochastic model is then employed to generate a large simulation box with modeled morphology and transfer integrals, which can fully capture the percolative charge transport. Finally, the charge-carrier mobility in this simulation box is calculated by solving a master equation, yielding a mobility function depending on temperature, carrier concentration, and electric field. We demonstrate the procedure for hole transport in two important molecular semiconductors, α -NPD and TCTA. In contrast to a previous study, we conclude that spatial correlations in the energy disorder are unimportant for α -NPD. We apply our mobility model to two types of hole-only α -NPD devices and find that the experimental temperature-dependent current density-voltage characteristics of all devices can be well described by only slightly decreasing the simulated energy disorder strength.
Ab initio Nuclear structure Theory with chiral two- plus three-nucleon interactions
Low-energy nuclear theory has entered an era of ab initio nuclear structure and reaction calculations based on input from QCD. One of the most promising paths from QCD to nuclear observables employs Hamiltonians constructed within chiral effective field theory as consistent starting point for precise ab initio nuclear structure and reaction studies. However, the full inclusion of chiral two- plus three-nucleon (NN+3N) interactions in exact and approximate many-body calculations still poses a formidable challenge. We discuss recent developments towards this goal, ranging from consistent Similarity Renormalization Group evolutions of NN+3N Hamiltonians to large-scale ab initio calculations for ground states and spectra in the Importance-Truncated No-Core Shell Model with full 3N interactions. We highlight recent achievements and discuss open issues and future perspectives for nuclear structure theory with QCD-based interactions. Moreover, we discuss successful steps towards merging ab initio structure and reaction theory and show applications to low-energy reactions in the p-shell relevant for astrophysics.
Exploring the free energy surface using ab initio molecular dynamics
Samanta, Amit; Morales, Miguel A.; Schwegler, Eric
2016-04-01
Efficient exploration of configuration space and identification of metastable structures in condensed phase systems are challenging from both computational and algorithmic perspectives. In this regard, schemes that utilize a set of pre-defined order parameters to sample the relevant parts of the configuration space [L. Maragliano and E. Vanden-Eijnden, Chem. Phys. Lett. 426, 168 (2006); J. B. Abrams and M. E. Tuckerman, J. Phys. Chem. B 112, 15742 (2008)] have proved useful. Here, we demonstrate how these order-parameter aided temperature accelerated sampling schemes can be used within the Born-Oppenheimer and the Car-Parrinello frameworks of ab initio molecular dynamics to efficiently and systematically explore free energy surfaces, and search for metastable states and reaction pathways. We have used these methods to identify the metastable structures and reaction pathways in SiO2 and Ti. In addition, we have used the string method [W. E, W. Ren, and E. Vanden-Eijnden, Phys. Rev. B 66, 052301 (2002); L. Maragliano et al., J. Chem. Phys. 125, 024106 (2006)] within the density functional theory to study the melting pathways in the high pressure cotunnite phase of SiO2 and the hexagonal closed packed to face centered cubic phase transition in Ti.
Exploring the free energy surface using ab initio molecular dynamics.
Samanta, Amit; Morales, Miguel A; Schwegler, Eric
2016-04-28
Efficient exploration of configuration space and identification of metastable structures in condensed phase systems are challenging from both computational and algorithmic perspectives. In this regard, schemes that utilize a set of pre-defined order parameters to sample the relevant parts of the configuration space [L. Maragliano and E. Vanden-Eijnden, Chem. Phys. Lett. 426, 168 (2006); J. B. Abrams and M. E. Tuckerman, J. Phys. Chem. B 112, 15742 (2008)] have proved useful. Here, we demonstrate how these order-parameter aided temperature accelerated sampling schemes can be used within the Born-Oppenheimer and the Car-Parrinello frameworks of ab initio molecular dynamics to efficiently and systematically explore free energy surfaces, and search for metastable states and reaction pathways. We have used these methods to identify the metastable structures and reaction pathways in SiO2 and Ti. In addition, we have used the string method [W. E, W. Ren, and E. Vanden-Eijnden, Phys. Rev. B 66, 052301 (2002); L. Maragliano et al., J. Chem. Phys. 125, 024106 (2006)] within the density functional theory to study the melting pathways in the high pressure cotunnite phase of SiO2 and the hexagonal closed packed to face centered cubic phase transition in Ti. PMID:27131525
Ab initio simulations on rutile-based titania nanowires
Zhukovskii, Yu F.; Evarestov, R. A.
2012-08-01
The rod symmetry groups for monoperiodic (1D) nanostructures have been applied for construction of models for bulk-like TiO2 nanowires (NWs) cut from a rutile-based 3D crystal along the chosen [001] and [110] directions of crystallographic axes. In this study, we have considered nanowires described by both the Ti-atom centered rotation axes as well as the hollow site centered axes passing through the interstitial positions between the Ti and O atoms closest to the axes. The most stable [001]-oriented TiO2 NWs with rhombic cross sections are found to display the energetically preferable {110} facets only while the nanowires with quasi-square sections across the [110] axis are formed by the alternating { 1bar 10 } and {001} facets. For simulations on rutile-based nanowires possessing different diameters for each NW type, we have performed large-scale ab initio Density Functional Theory (DFT) and hybrid DFT-Hartree Fock (DFT-HF) calculations with total geometry optimization within the Generalized Gradient Approximation (GGA) in the form of the Perdew-Becke-Ernzenhof (PBE) exchange-correlation functionals (PBE and PBE0, respectively), using the formalism of linear combination of localized atomic functions (LCAO). We have simulated both structural and electronic properties of TiO2 NWs depending both on orientation and position of symmetry axes as well as on diameter and morphology of nanowires.
Arai, K.; Aoyama, S.; Suzuki, Y.; Descouvemont, P.; Baye, D. [Division of General Education, Nagaoka National College of Technology, 888 Nishikatakai, Nagaoka, Niigata, 940-8532 (Japan); Center for Academic Information Service, Niigata University, Niigata 950-2181 (Japan); Department of Physics, Niigata University, Niigata 950-2181, Japan and RIKEN Nishina Center, Wako 351-0198 (Japan); Physique Nucleaire Theorique et Physique Mathematique, C.P.229, Universite Libre de Bruxelles, B 1050 Brussels (Belgium); Physique Quantique, CP165/82, Universite Libre de Bruxelles, B-1050 Brussels (Belgium)
2012-11-12
The {sup 2}H(d,p){sup 3}H, {sup 2}H(d,n){sup 3}He, and {sup 2}H(d,{gamma}){sup 4}He reactions at low energies are studied with realistic nucleon-nucleon interactions in an ab initio approach. The obtained astrophysical S-factors are all in very good agreement with experiment. The most important channels for both transfer and radiative capture are all found to dominate thanks to the tensor force.
Ganster, P
2004-10-15
A calcium aluminosilicate glass of molar composition 67 % SiO{sub 2} - 12 % Al{sub 2}O{sub 3} - 21 % CaO was modelled by classical and ab initio molecular dynamics. The size effect study in classical MD shows that the systems of 100 atoms are more ordered than the larger ones. These effects are mainly due to the 3-body terms in the empirical potentials. Nevertheless, these effects are small and the structures generated are in agreement with experimental data. In such kind of glass, we denote an aluminium avoidance and an excess of non bridging oxygens which can be compensated by tri coordinated oxygens. When the dynamics of systems of 100 and 200 atoms is followed by ab initio MD, some local arrangements occurs (bond length, angular distributions). Thus, more realistic vibrational properties are obtained in ab initio MD. The modelling of thin films shows that aluminium atoms extend to the most external part of the surface and they are all tri-coordinated. Calcium atoms are set in the sub layer part of the surface and they produce a depolymerization of the network. In classical MD, tri-coordinated aluminium atoms produce an important electric field above the surface. With non bridging oxygens, they constitute attractive sites for single water molecules. (author)
Altun, Ahmet; Yokoyama, Shozo; Morokuma, Keiji
2008-01-01
We have investigated photoabsorption spectra of bovine rhodopsin and its mutants (E122Q and E113Q) by hybrid quantum mechanical/molecular mechanical (QM/MM) calculations as well as retinal in vacuo by pure QM calculations, employing multireference (MR) ab initio and TD-B3LYP methods. The sophisticated MR-SORCI+Q and MRCISD+Q methods extrapolated with respect to adopted approximations can reproduce the experimental absorption maxima of retinal very well. The relatively inexpensive MR-DDCI2+Q m...
Ab initio study of the complexes of first-row transition-metal ions with CH, CH2, and CH3
李吉海; 冯大诚; 冯圣玉
1999-01-01
The geometries and bonding characteristics of the complexes of the first-row transition-metal ions with CH, CH2 and CH3 were investigated by ab initio molecular orbital theory. MCH+ and MCH2+ are linear and coplanar, re spectively. Both of them are with obvious treble or double bond characteristics, but these multiple bonds are mostly "im perfect". The calculated bond dissociation energies of C--M+, C=M+ and C≡M+ are mostly close to the experi mental values, and appear in similar periodic trends from Sc to Zn.
Mayo, Michael L.; Ray, Asok K.
2003-01-01
Ab initio self-consistent total energy calculations using second order Moller-Plesset perturbation theory and Hay-Wadt effective core potentials for gallium and arsenic have been used to investigate the chemisorption of atomic oxygen on the Ga-rich GaAs (100) (2 x 1) and beta(4 x 2) surfaces. Finite sized hydrogen saturated clusters with the experimental zinc-blende lattice constant of 5.654 angstroms and the energy optimized surface Ga dimer bond length of 2.758 angstroms have been used to m...
Lumbroso, H.; Liégeois, C.; Pappalardo, G. C.; Grassi, A.
From the ab initio molecular energies of the possible conformers and from a classical dipole moment analysis of 2-oxopyrrolidin-l-ylacetamide (μ = 4.02 D in dioxan at 30.0°C), the preferred conformation in solution of this novel nootropic agent has been determined. The exocyclic N-CH 2 bond is rotated in one sense by 90° and the exocyclic CH 2-C bond rotated in the same sense by 120° from the "planar" ( OO)- cis conformation. The structures of the two enantiomers in solution differ from that of the crystalline molecule.
Palmer, Michael H.; Hoffmann, Søren Vrønning; Jones, Nykola C.;
2011-01-01
The Rydberg states in the vacuum ultraviolet photoabsorption spectrum of 1,2,3-triazole have been measured and analyzed with the aid of comparison to the UV valence photoelectron ionizations and the results of ab initio configuration interaction (CI) calculations. Calculated electronic ionization......-cluster (singles, doubles, and triples) and complete active space self-consistent field procedures agree well with experimental values. Variations in bond lengths with the electronic state are discussed. The lowest energy UV band (∼5.5–6.5 eV) is assigned to three electronically excited states and demonstrates...
Härkönen, Ville J.; Karttunen, Antti J.
2016-01-01
The lattice thermal conductivity of silicon clathrate framework Si$_{23}$ and two Zintl clathrates, [Si$_{19}$P$_{4}$]Cl$_{4}$ and Na$_{4}$[Al$_{4}$Si$_{19}$], is investigated by using an iterative solution of the linearized Boltzmann transport equation (BTE) in conjuntion with ab initio lattice dynamical techniques. At 300 K, the lattice thermal conductivities for Si$_{23}$, [Si$_{19}$P$_{4}$]Cl$_{4}$, and Na$_{4}$[Al$_{4}$Si$_{19}$] were found to be 43 W/(m K), 25 W/(m K), and 2 W/(m K), re...
Duijnen, P.Th. van; Thole, B.Th.; Broer, Ria; Nieuwpoort, W. C.
1980-01-01
Ab initio MO calculations, using both minimal (STO-3G) and extended (Roos-Siegbahn) basis sets are reported for the systems methanethiol-imidazole, methanethiol-imidazole-formaldehyde, and methanethiol-imidazole-formamide, which, together with a point-change representation of a long α-helix, form models for the active site of papain. It is shown that the large electric field exerted by the helix in the active-site region is responsible for the presence of the essential residues Cys 25 and His...
Ab initio lattice dynamics of complex structures
Voss, Johannes
2008-01-01
In this thesis, density functional theory is applied in a study of thermodynamic properties of so-called complex metal hydrides, which are promising materials for hydrogen storage applications. Since the unit cells of these crystals can be relatively large with many symmetrically inequivalent...... atomic coordinates, we have developed a new numerical optimization scheme, which allows for a fast convergence of the coordinate relaxation. Moreover, a method for the efficient calculation of phonon frequencies has been developed, which is based on a combination of density functional theory calculations...... have been studied, showing that the mobility of hydrogen is limited by high energetic barriers in the intermediate decomposition product Na3AlH6 in particular, and that the effect of titanium as a dopant on the dynamics is negligible. The presented methods and studies demonstrate possibilities for a...
Challenges for ab initio defect modeling
Deak, Peter [U. Bremen, Bremen Center for Computational Materials Science, POB 330440, D-28334 Bremen (Germany)], E-mail: deak@bccms.uni-bremen.de; Aradi, Balint; Frauenheim, Thomas [U. Bremen, Bremen Center for Computational Materials Science, POB 330440, D-28334 Bremen (Germany); Gali, Adam [Budapest University of Technology and Economics, Dept. Atomic Physics, H-1521 Budapest (Hungary)
2008-12-05
Supercell calculations using density functional theory with local or semi-local exchange functionals have proved to be a very successful tool in defect engineering, apparently leading to some degree of overconfidence lately. With a case study on selected defects, we demonstrate that the approximations involved in these functionals lead not only to an underestimation of the gap but also to related errors in the total energy difference of two defect configurations (be these in the same or different charge states). Sometimes the error is so serious, that even the common expectation: 'DFT provides the correct ground state' is refuted. We also demonstrate, that semi-empirical hybrid exchange functionals, which reproduce the correct band gap, give good total energy differences even in these cases. In such calculations, the position of Kohn-Sham gap levels with respect to the band edges gives a good estimation of the vertical ionization energy. Based on that, we propose a simple correction scheme for checking the necessity of extended total energy calculations with a hybrid functional.
Equations of state of heavy metals: ab initio approaches
The determination of equations of states of heavy metals through ab initio calculation, i.e. without any adjustable parameter, allows to access to pressure and temperature thermodynamic conditions sometimes inaccessible to experiment. To perform such calculations, density functional theory (DFT) is a good starting point: when electronic densities are homogeneous enough, the local density approximation (LDA) remarkably accounts for thermodynamic properties of heavy metals, such as tantalum, or the light actinides, as well for static properties - equilibrium volume, elastic constants - as for dynamical quantities like phonon spectra. For heavier elements, like neptunium or plutonium, relativistic effects and strong electronic interactions must be taken into account, which requires more sophisticated theoretical approaches. (authors)
Ab initio calculation of tensile strength in iron
Friák, Martin; Šob, Mojmír; Vitek, V.
2003-01-01
Roč. 83, 31-34 (2003), s. 3529-3537. ISSN 1478-6435. [Multiscale Materials Modelling: Working Theory for Industry /1./. London, 17.06.2002-20.06.2002] R&D Projects: GA AV ČR IAA1041302; GA ČR GA202/03/1351; GA MŠk OC 523.90 Institutional research plan: CEZ:AV0Z2041904 Keywords : ab initio calculations * electronic structure * theoretical tensile strength Subject RIV: BM - Solid Matter Physics ; Magnetism
Ab initio vibrational and dielectric properties of Y V O
Vali, R.
2009-10-01
For the yttrium orthovanadate Y V O with a tetragonal zircon-type structure, the first complete set of Raman-active and IR-active phonon modes has been calculated using ab initio density functional perturbation theory. The calculated IR reflectivity spectra are in good agreement with available experimental data. We report the calculated frequencies of three Raman-active modes that could not be detected experimentally and a new assignment of the experimental Raman data. The contributions of each IR-active phonon modes to static dielectric tensor have been determined.
Accelerating Ab Initio Nuclear Physics Calculations with GPUs
Potter, Hugh; Maris, Pieter; Sosonkina, Masha; Vary, James; Binder, Sven; Calci, Angelo; Langhammer, Joachim; Roth, Robert; Çatalyürek, Ümit; Saule, Erik
2014-01-01
This paper describes some applications of GPU acceleration in ab initio nuclear structure calculations. Specifically, we discuss GPU acceleration of the software package MFDn, a parallel nuclear structure eigensolver. We modify the matrix construction stage to run partly on the GPU. On the Titan supercomputer at the Oak Ridge Leadership Computing Facility, this produces a speedup of approximately 2.2x - 2.7x for the matrix construction stage and 1.2x - 1.4x for the entire run.
Ab initio structure determination via powder X-ray diffraction
Digamber G Porob; T N Guru Row
2001-10-01
Structure determination by powder X-ray diffraction data has gone through a recent surge since it has become important to get to the structural information of materials which do not yield good quality single crystals. Although the method of structure completion when once the starting model is provided is facile through the Rietveld refinement technique, the structure solution ab initio os still not push-button technology. In this article a survey of the recent development in this area is provided with an illustration of the structure determination of -NaBi3V2O10.
Vibók, Á.; Halász, G. J.; Suhai, S.; Hoffman, D. K.; Kouri, D. J.; Baer, M.
2006-01-01
In this article we present the first ab initio study of the conical intersections (cis) and their electronic nonadiabatic coupling terms (NACTs) for the {N,H2} system. Efforts were made to reveal the location of cis between the two lower, 1A'2 and 2A'2 states—to be designated as (1,2) cis—and the cis between the two upper, 2A'2 and 3A'2 states—to be designated as the (2,3) cis—of this system. We found that these cis are located along the collinear {NHH) arrangement. The study is carried out by analyzing two-state magnitudes such as the (1,2) and (2,3) adiabatic-to-diabatic transformation angles (known also as the mixing angles) and the corresponding topological phases (known also as the Berry phases or the Longuet-Higgins phases). In addition, a detailed three-state study is carried out. Here the emphasis is on driving the diagonal elements of the topological D matrix and analyzing situations for which the corresponding nonadiabatic coupling matrix is quantized. The reliability of two-state results is carefully examined by comparing them with corresponding outcomes derived for the three-state study. In addition we also calculated the potential-energy surfaces related to the two lower states and studied to what extent they are affected by the (1,2) ci. The results obtained in this treatment were found to be in full agreement with the NACT's calculations.
Photostructural changes—the hallmark of non-crystalline chalcogenides—are in essence the basis of a number of photoinduced effects, i.e., changes of their physical properties, which are exploited in a variety of applications, especially in photonics and optoelectronics. Despite the vast number of investigations of photostructural changes, there is currently lack of systematic studies on how the thermal history, which affects glass structure, modifies the extent of photostructural changes. In this article, we study the role of thermal history on photostructural changes in glassy As15S85. This particular sulfur-rich composition has been chosen based on the colossal photostructural response it exhibits under near-band gap light irradiation, which inherently originates from its nanoscale phase-separated nature. To control the thermal history, the glass was quenched to various temperatures and each of these quenched products was annealed under four different conditions. Off-resonant Raman scattering was used to study the equilibrium study of each product. Structural changes of interest involve changes of the sulfur atoms participating into S8 rings and Sn chains. Their ratio was found to depend on quenching/annealing conditions. Near-band gap light was used to perturb the rings-to-chain ratio and at the same time to record these changes through Raman scattering, revealing an intricate behavior of photostructural changes. Ab initio calculations were employed to determine the stability of various sulfur clusters/molecules thus aiding the correlation of the particular photo-response of glassy As15S85 with its structural constituents
Kerscher, Tobias C.; Mueller, Stefan [Technische Universitaet Hamburg-Harburg, Institut fuer Keramische Hochleistungswerkstoffe, Denickestr. 15, 21073 Hamburg (Germany)
2011-07-01
Understanding the adsorbate-induced segregation of alloy surfaces is essential for catalytic surfaces. An interesting example is the C/Pt{sub 25}Rh{sub 75}(100) system: In comparison to the clean surface, even a small amount of carbon impurities leads to a considerable decrease in the Pt concentration of the top layer. Previously, our first principles study of the coupled surface-adsorbate system was restricted to T=0 K, where density functional theory shows that this change in segregation is driven by the antagonism between the segregation energies of Pt and Rh, and their binding energies to C. For the clean surface both experiment and our first-principles cluster-expansion study quantitatively agree on a Pt enrichment in the first layer and a depletion underneath; for even a small amount of carbon impurities a considerable decrease in the Pt concentration of the top layer is observed. Our T=0 K density functional theory studies show that this change is driven by the antagonism between the segregation energies of Pt and Rh, and their binding energies to C. Now, we use an ab-initio-based cluster expansion in the framework of the UNCLE code to investigate the T>0 K temperature behaviour of the system by means of Monte Carlo simulations. We discuss the substitutional ordering of both the carbon adsorbate layer and the four topmost surface layers of Pt{sub 25}Rh{sub 75}(100) as a function of temperature and concentration. Our predictions are compared to experimental data in a quantitative manner. Density-functional calculations provide the energetics for the construction of the position-dependent interactions for the cluster-expansion Hamiltonian. The properties of the system are found to be controlled by two competing effects: the different segregation energies of Pt and Rh, and their individual binding energies to C. We discuss the substitutional ordering of the system as a function of C-coverage.
Graphical abstract: Indole is the chromophore of tryptophan. Do the various conformations of the alanyl side-chain influence the properties of the low-lying excited states? We answer this question by means of accurate ab initio calculations on several tryptophan conformer in the gas phase, and show, for instance, that the polarity of the electronic states strongly depends on the alanyl conformation. - Abstract: The complex electronic structure of gas phase tryptophan is investigated by ab initio methods and time-dependent density functional theory (TD-DFT), as a function of the conformations of the alanyl side chain. Energies, oscillator strengths, and dipole moments of the four lowest lying excited states are determined from multi-configurational (CASSCF/MS-CASPT2) calculations using a rather large basis set. While the second excited state always carries the largest oscillator strength for all conformers investigated in the present work, we find that the permanent dipole moment is strongly conformer dependent. TD-DFT results are barely useful for the proper characterization of the excited states.
We present an analysis of the spin–rotation and absolute shielding constants of XF6 molecules (X = S, Se, Te, Mo, W) based on ab initio coupled cluster and four-component relativistic density-functional theory (DFT) calculations. The results show that the relativistic contributions to the spin–rotation and shielding constants are large both for the heavy elements as well as for the fluorine nuclei. In most cases, incorporating the computed relativistic corrections significantly improves the agreement between our results and the well-established experimental values for the isotropic spin–rotation constants and their anisotropic components. This suggests that also for the other molecules, for which accurate and reliable experimental data are not available, reliable values of spin–rotation and absolute shielding constants were determined combining ab initio and relativistic DFT calculations. For the heavy nuclei, the breakdown of the relationship between the spin–rotation constant and the paramagnetic contribution to the shielding constant, due to relativistic effects, causes a significant error in the total absolute shielding constants
Pozzi, C. G.; Fantoni, A. C.; Goeta, A. E.; Wilson, C. C.; Autino, J. C.; Punte, G.
2005-10-01
The molecular and crystal structures of the title compound have been determined from a single crystal neutron diffraction experiment at 100 K. A comparison between the main geometrical features and related properties of the in-crystal and the ab initio optimized free molecule structures has shown that crystal packing induces a significant distortion in the molecular geometry. Packing instead would only have a moderate effect on the observed intramolecular resonance assisted hydrogen bond. Supermolecular ab initio molecular orbital calculations have been performed on the six different dimers one molecule forms with its nine nearest neighbours. The obtained results clearly show that dispersion contributions dominate in the most strongly interacting dimers, in good qualitative accord with the predictions made by using different empirical potentials. A qualitative description of the most prominent inductive effects determined from the electron density deformation upon dimer formation is presented. Topological analyses of the dimers charge densities have been performed in the framework of the Bader's AIM theory and all the intermolecular bond critical points have been identified. An attempt to determine some of the interaction energies only from topological quantities made evident the practical limitations of such an approach.
Abadias, G.; Kanoun, M. B.; Goumri-Said, S.; Koutsokeras, L.; Dub, S. N.; Djemia, Ph.
2014-10-01
The structure, phase stability, and mechanical properties of ternary alloys of the Zr-Ta-N system are investigated by combining thin-film growth and ab initio calculations. Zr1-xTaxN films with 0≤x≤1 were deposited by reactive magnetron cosputtering in Ar +N2 plasma discharge and their structural properties characterized by x-ray diffraction. We considered both ordered and disordered alloys, using supercells and special quasirandom structure approaches, to account for different possible metal atom distributions on the cation sublattice. Density functional theory within the generalized gradient approximation was employed to calculate the electronic structure as well as predict the evolution of the lattice parameter and key mechanical properties, including single-crystal elastic constants and polycrystalline elastic moduli, of ternary Zr1-xTaxN compounds with cubic rocksalt structure. These calculated values are compared with experimental data from thin-film measurements using Brillouin light scattering and nanoindentation tests. We also study the validity of Vegard's empirical rule and the effect of growth-dependent stresses on the lattice parameter. The thermal stability of these Zr1-xTaxN films is also studied, based on their structural and mechanical response upon vacuum annealing at 850 °C for 3 h. Our findings demonstrate that Zr1-xTaxN alloys with Ta fraction 0.51⩽x⩽0.78 exhibit enhanced toughness, while retaining high hardness ˜30 GPa, as a result of increased valence electron concentration and phase stability tuning. Calculations performed for disordered or ordered structures both lead to the same conclusion regarding the mechanical behavior of these nitride alloys, in agreement with recent literature findings [H. Kindlund, D. G. Sangiovanni, L. Martinez-de-Olcoz, J. Lu, J. Jensen, J. Birch, I. Petrov, J. E. Greene, V. Chirita, and L. Hultman, APL Materials 1, 042104 (2013), 10.1063/1.4822440].
AB INITIO SIMULATIONS FOR MATERIAL PROPERTIES ALONG THE JUPITER ADIABAT
We determine basic thermodynamic and transport properties of hydrogen-helium-water mixtures for the extreme conditions along Jupiter's adiabat via ab initio simulations, which are compiled in an accurate and consistent data set. In particular, we calculate the electrical and thermal conductivity, the shear and longitudinal viscosity, and diffusion coefficients of the nuclei. We present results for associated quantities like the magnetic and thermal diffusivity and the kinematic shear viscosity along an adiabat that is taken from a state-of-the-art interior structure model. Furthermore, the heat capacities, the thermal expansion coefficient, the isothermal compressibility, the Grüneisen parameter, and the speed of sound are calculated. We find that the onset of dissociation and ionization of hydrogen at about 0.9 Jupiter radii marks a region where the material properties change drastically. In the deep interior, where the electrons are degenerate, many of the material properties remain relatively constant. Our ab initio data will serve as a robust foundation for applications that require accurate knowledge of the material properties in Jupiter's interior, e.g., models for the dynamo generation.
Ab Initio Thermodynamic Model for Magnesium Carbonates and Hydrates
Chaka, Anne M.; Felmy, Andrew R.
2014-03-28
An ab initio thermodynamic framework for predicting properties of hydrated magnesium carbonate minerals has been developed using density-functional theory linked to macroscopic thermodynamics through the experimental chemical potentials for MgO, water, and CO2. Including semiempirical dispersion via the Grimme method and small corrections to the generalized gradient approximation of Perdew, Burke, and Ernzerhof for the heat of formation yields a model with quantitative agreement for the benchmark minerals brucite, magnesite, nesquehonite, and hydromagnesite. The model shows how small differences in experimental conditions determine whether nesquehonite, hydromagnesite, or magnesite is the result of laboratory synthesis from carbonation of brucite, and what transformations are expected to occur on geological time scales. Because of the reliance on parameter-free first principles methods, the model is reliably extensible to experimental conditions not readily accessible to experiment and to any mineral composition for which the structure is known or can be hypothesized, including structures containing defects, substitutions, or transitional structures during solid state transformations induced by temperature changes or processes such as water, CO2, or O2 diffusion. Demonstrated applications of the ab initio thermodynamic framework include an independent means to evaluate differences in thermodynamic data for lansfordite, predicting the properties of Mg analogs of Ca-based hydrated carbonates monohydrocalcite and ikaite which have not been observed in nature, and an estimation of the thermodynamics of barringtonite from the stoichiometry and a single experimental observation.
Three-cluster dynamics within an ab initio framework
Quaglioni, S; Navrátil, P
2013-01-01
We introduce a fully antisymmetrized treatment of three-cluster dynamics within the ab initio framework of the no-core shell model/resonating-group method (NCSM/RGM). Energy-independent non-local interactions among the three nuclear fragments are obtained from realistic nucleon-nucleon interactions and consistent ab initio many-body wave functions of the clusters. The three-cluster Schr\\"odinger equation is solved with bound-state boundary conditions by means of the hyperspherical-harmonic method on a Lagrange mesh. We discuss the formalism in detail and give algebraic expressions for systems of two single nucleons plus a nucleus. Using a soft similarity-renormalization-group evolved chiral nucleon-nucleon potential, we apply the method to an $^4$He+$n+n$ description of $^6$He and compare the results to experiment and to a six-body diagonalization of the Hamiltonian performed within the harmonic-oscillator expansions of the NCSM. Differences between the two calculations provide a measure of core ($^4$He) pola...
Ab Initio Protein Structure Prediction Using Pathway Models
Christopher Bystroff
2006-04-01
Full Text Available Ab initio prediction is the challenging attempt to predict protein structures based only on sequence information and without using templates. It is often divided into two distinct sub-problems: (a the scoring function that can distinguish native, or native-like structures, from non-native ones; and (b the method of searching the conformational space. Currently, there is no reliable scoring function that can always drive a search to the native fold, and there is no general search method that can guarantee a significant sampling of near-natives. Pathway models combine the scoring function and the search. In this short review, we explore some of the ways pathway models are used in folding, in published works since 2001, and present a new pathway model, HMMSTR-CM, that uses a fragment library and a set of nucleation/propagation-based rules. The new method was used for ab initio predictions as part of CASP5. This work was presented at the Winter School in Bioinformatics, Bologna, Italy, 10Ã¢Â€Â“14 February 2003.
Unified ab initio approaches to nuclear structure and reactions
Navrátil, Petr; Quaglioni, Sofia; Hupin, Guillaume; Romero-Redondo, Carolina; Calci, Angelo
2016-05-01
The description of nuclei starting from the constituent nucleons and the realistic interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of the nuclear forces, with two-, three- and possibly higher many-nucleon components, one faces the quantum-mechanical many-nucleon problem governed by an interplay between bound and continuum states. In recent years, significant progress has been made in ab initio nuclear structure and reaction calculations based on input from QCD-employing Hamiltonians constructed within chiral effective field theory. After a brief overview of the field, we focus on ab initio many-body approaches—built upon the no-core shell model—that are capable of simultaneously describing both bound and scattering nuclear states, and present results for resonances in light nuclei, reactions important for astrophysics and fusion research. In particular, we review recent calculations of resonances in the 6He halo nucleus, of five- and six-nucleon scattering, and an investigation of the role of chiral three-nucleon interactions in the structure of 9Be. Further, we discuss applications to the 7Be {({{p}},γ )}8{{B}} radiative capture. Finally, we highlight our efforts to describe transfer reactions including the 3H{({{d}},{{n}})}4He fusion.
Unified ab initio approaches to nuclear structure and reactions
Navratil, Petr; Hupin, Guillaume; Romero-Redondo, Carolina; Calci, Angelo
2016-01-01
The description of nuclei starting from the constituent nucleons and the realistic interactions among them has been a long-standing goal in nuclear physics. In addition to the complex nature of the nuclear forces, with two-, three- and possibly higher many-nucleon components, one faces the quantum-mechanical many-nucleon problem governed by an interplay between bound and continuum states. In recent years, significant progress has been made in {\\em ab initio} nuclear structure and reaction calculations based on input from QCD-employing Hamiltonians constructed within chiral effective field theory. After a brief overview of the field, we focus on ab initio many-body approaches - built upon the No-Core Shell Model - that are capable of simultaneously describing both bound and scattering nuclear states, and present results for resonances in light nuclei, reactions important for astrophysics and fusion research. In particular, we review recent calculations of resonances in the $^6$He halo nucleus, of five- and six...
Ab initio dynamics of the cytochrome P450 hydroxylation reaction
Elenewski, Justin E.; Hackett, John C, E-mail: jchackett@vcu.edu [Department of Physiology and Biophysics and The Massey Cancer Center, School of Medicine, Virginia Commonwealth University, 401 College Street, Richmond, Virginia 23219-1540 (United States)
2015-02-14
The iron(IV)-oxo porphyrin π-cation radical known as Compound I is the primary oxidant within the cytochromes P450, allowing these enzymes to affect the substrate hydroxylation. In the course of this reaction, a hydrogen atom is abstracted from the substrate to generate hydroxyiron(IV) porphyrin and a substrate-centered radical. The hydroxy radical then rebounds from the iron to the substrate, yielding the hydroxylated product. While Compound I has succumbed to theoretical and spectroscopic characterization, the associated hydroxyiron species is elusive as a consequence of its very short lifetime, for which there are no quantitative estimates. To ascertain the physical mechanism underlying substrate hydroxylation and probe this timescale, ab initio molecular dynamics simulations and free energy calculations are performed for a model of Compound I catalysis. Semiclassical estimates based on these calculations reveal the hydrogen atom abstraction step to be extremely fast, kinetically comparable to enzymes such as carbonic anhydrase. Using an ensemble of ab initio simulations, the resultant hydroxyiron species is found to have a similarly short lifetime, ranging between 300 fs and 3600 fs, putatively depending on the enzyme active site architecture. The addition of tunneling corrections to these rates suggests a strong contribution from nuclear quantum effects, which should accelerate every step of substrate hydroxylation by an order of magnitude. These observations have strong implications for the detection of individual hydroxylation intermediates during P450 catalysis.
Graphical abstract: - Highlights: • FP-LAPW method has been used to compute the solid state properties of AgGaX2 (X = S, Se, Te). • Electronic and optical properties reported with recently developed mBJ potential. • Thermal expansion, heat capacity, Debye temperature, entropy and Grüneisen parameter were evaluated. • Hardness was calculated for the first time at different temperature and pressure. - Abstract: We have performed ab initio calculations for the structural, electronic, optical, elastic and thermal properties of the silver gallium dichalcogenides (AgGaX2: X = S, Se, Te). In this study, we have used the accurate full potential linearized augmented plane wave (FP-LAPW) method to find the equilibrium structural parameters and to compute the six elastic constants (C11, C12, C13, C33, C44 and C66). We have reported electronic and optical properties with the recently developed density functional theory of Tran and Blaha, and this theory is used along with the Wu-Cohen generalized gradient approximation (WC-GGA) for the exchange-correlation potential. Furthermore, optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients and optical conductivities were calculated for photon energies up to 40 eV. The thermodynamical properties such as thermal expansion, heat capacity, debye temperature, entropy, Grüneisen parameter and bulk modulus were calculated employing the quasi-harmonic Debye model at different temperatures (0–900 K) and pressures (0–8 GPa) and the silent results were interpreted. Hardness of the materials was calculated for the first time at different temperatures and pressures
Li, Zhe; Wu, Yinuo; Feng, Ling-Jun; Wu, Ruibo; Luo, Hai-Bin
2014-12-01
Phosphodiesterases (PDEs) are the sole enzymes hydrolyzing the important second messengers cGMP and cAMP and have been identified as therapeutic targets for several diseases. The most successful examples are PDE5 inhibitors (i.e., sildenafil and tadalafil), which have been approved for the treatment of male erectile dysfunction and pulmonary hypertension. However, the side effects mostly due to nonselective inhibition toward other PDE isoforms, set back the clinical usage of PDE5 inhibitors. Until now, the exact catalytic mechanism of the substrate cGMP by PDE5 is still unclear. Herein, the first computational study on the catalytic hydrolysis mechanism of cGMP for PDE5 (catalytic domain) is performed by employing the state-of-the-art ab initio quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations. Our simulations show a SN2 type reaction procedure via a highly dissociated transition state with a reaction barrier of 8.88 kcal/mol, which is quite different from the previously suggested hydrolysis mechanism of cAMP for PDE4. Furthermore, the subsequent ligand exchange and the release of the product GMP have also been investigated by binding energy analysis and MD simulations. It is deduced that ligand exchange would be the rate-determining step of the whole reaction, which is consistent with many previous experimental results. The obtained mechanistic insights should be valuable for not only the rational design of more specific inhibitors toward PDE5 but also understanding the general hydrolysis mechanism of cGMP-specific PDEs. PMID:26583228
The results of comprehensive theoretical and experimental study of binary GaN/AlN multi-quantum well (MQW) systems oriented along polar c-direction of their wurtzite structure are presented. A series of structures with quantum wells and barriers of various thicknesses were grown by plasma-assisted molecular-beam epitaxy and characterized by x-ray diffraction and transmission electron microscopy. It was shown that in general the structures of good quality were obtained, with the defect density decreasing with increasing quantum well thickness. The optical transition energies in these structures were investigated comparing experimental measurements with ab initio calculations of the entire GaN/AlN MQW structure depending on the QW widths and strains, allowing for direct determination of the energies of optical transitions and the electric fields in wells/barriers by electric potential double averaging procedure. Photoluminescence (PL) measurements revealed that the emission efficiency as well as the shape of luminescence spectra correlated well with their structural quality. Additionally, due to the Quantum-Confined Stark Effect, the emission energy decreased by over 1 eV for quantum well thicknesses increasing from 1 nm up to 6 nm, and this effect was accompanied by the drastic drop of the PL efficiency. The experimental results are consistent with theoretical models. Comparison of experimental data obtained by a number of different characterization techniques with the density functional theory results received on the same geometry structure allowed to prove directly the theoretical models and to determine the polarization and the oscillator strengths in the AlN/GaN nitride systems for the first time