Pseudopotential Scheme Research Articles

First-principle calculations of the structure and elastic properties of GaAs under pressure

A first-principles plane-wave method with the ultrasoft pseudopotential scheme in the framework of density functional theory is performed to calculate the lattice parameters, the bulk modulus B0 and its pressure derivative B0' of the zinc-blende GaAs (ZB–GaAs), rocksalt GaAs (RS–GaAs), CsCl–GaAs, NiAs–GaAs, and wurtzite GaAs (WZ–GaAs). We also calculate the phase transition pressures between different phases, Debye temperatures, and the anisotropies. Our results are consistent with other theoretical results.

Implementation of a DFT-Based Method for the Calculation of the Zeeman g-Tensor in Periodic Systems with the Use of Numerical and Slater-Type Atomic Orbitals

The Zeeman g-tensor parameterizes the interaction of an effective electronic spin with the homogeneous external magnetic field in the electron paramagnetic resonance (EPR) experiment. In this article, we describe a Kohn-Sham DFT (KS DFT)-based implementation of the g-tensor for periodic systems. Our implementation can be used, for example, for the first-principles calculation of a g-tensor of paramagnetic defects in solids. Our approach is based on the method of Van Lenthe et al. in which the spin-orbital coupling is taken into account variationally. The method is implemented in the BAND program, a KS DFT implementation for periodic systems. The Bloch states are expanded in the basis of numerical and Slater-type atomic orbitals (NAOs/STOs). Our implementation does not rely on the frozen core approximation tacitly assumed in the pseudopotential schemes. The implementation is validated by calculating the g-tensor for small molecules as well as for paramagnetic defects in solids. In particular, we consider ozonide and hydrogen cyanide anion radicals in a KCl host crystal lattice.

Linear response approach to collective electronic excitations of solids and surfaces

We have developed a parallel computer program for the study of dynamic response of periodic systems. It computes the linear response of an interacting many-electron system from its ground-state electronic structures, which are obtained from ab initio band structure calculations in the plane-wave and pseudopotential scheme. As test examples, we applied this program to calculate the linear response of bulk aluminum and a beryllium monolayer. The excitation spectra show prominent plasmon resonances, which compare well with the available data and previous calculations. For surfaces or thin films, we found that removing periodicity perpendicular to the surface gives a more reliable description of the low-energy excitation spectra, especially in the long-wavelength limit. Program summary Program title: Dresponse Catalogue identifier: AECK_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AECK_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 49 098 No. of bytes in distributed program, including test data, etc.: 11 836 088 Distribution format: tar.gz Programming language: Fortran 90/MPI Computer: Any architecture with a Fortran 90 compiler Operating system: Any Has the code been vectorized or parallelized?: Yes RAM: 50 MB–2 GB per processor depending on system size Classification: 7.3 External routines: BLAS ( http://www.netlib.org/blas/), Lapack ( http://www.netlib.org/lapack/), MPI ( http://www-unix.mcs.anl.gov/mpi/), abinit (for ground-state calculations, http://www.abinit.org/) Nature of problem: The dynamic response of bulk and surface systems. It is usually dominated by collective electronic excitations (plasmons) at low-energy range. Solution method: The ground-state wavefunctions are obtained from ab initio density-functional calculation in the planewave and pseudopotential scheme [1]. The linear response theory combined with the time-dependent density functional theory is implemented for the investigation of excitation properties. Restrictions: The present version only handles 3D and 2D periodic systems. Unusual features: A mixing reciprocal/real-space basis is implemented in surface calculations in order to remove the intercell coupling in solving the Dyson equation. This treatment provides more reliable results, especially in the long-wavelength limit. Running time: The example included in the distribution takes a few minutes to complete.

Calculations of electronic structure, elastic properties for YxGa1−xN and band offsets of YxGa1−xN/ScyGa1−yN heterointerfaces

Using the pseudopotential scheme under the virtual crystal approximation combined with the Harrison bond-orbital model, we report a theoretical investigation of the electronic and elastic properties of YxGa1−xN ternary alloys in the zinc-blende structure. Besides, band offsets calculations for pseudomorphically strained YxGa1−xN/ScyGa1−yN interfaces have been performed on the basis of the model solid theory. Our results agree generally well with the available experimental and previously published theoretical data. The composition dependence of the selected features of YxGa1−xN such as energy band gaps, effective masses, elastic constants, bulk modulus, shear modulus, internal strain parameter, and valence and conduction band offsets has been examined. To the author's best knowledge, there had been no reported work on these properties for the material under load.

Structures and Phase Transition of GaAs under Pressure

A first-principles plane wave method with the ultrasoft pseudopotential scheme in the frame of the density functional theory (DFT) is performed to calculate the lattice parameters a and c, the bulk modulus B0 and its pressure derivative B′0 of the zinc-blende GaAs (ZB-GaAs), rocksalt GaAs (RS-GaAs), CsCl-GaAs, NiAs-GaAs and wurtzite GaAs (WZ-GaAs). Our results are consistent with the available experimental data and other theoretical results. We also calculate the phase transition pressures among these different phases. The results are satisfactory.

ELASTIC CONSTANTS AND ACOUSTIC WAVE VELOCITIES INCd1-xZnxTeMIXED CRYSTALS

Based on the pseudopotential scheme under the virtual crystal approximation that takes into account the effect of compositional disorder combined with the bond-orbital model of Harrison, the results of calculations of elastic constants and surface and bulk acoustic wave speeds of Cd1-xZnxTe mixed crystals in the zinc-blende structure are presented. The agreement between our results and known data, which are only available for CdTe and ZnTe is found to be reasonable.

Nonadiabatic couplings from time-dependent density functional theory. II. Successes and challenges of the pseudopotential approximation

We present extensive calculations of nonadiabatic couplings (NACs) between the electronically ground and excited states of molecules, using time-dependent density functional theory (TDDFT) within (modified) linear response [C. Hu et al. J. Chem. Phys. 127, 064103 (2007)]. Our approach is implemented in the pseudopotential framework, with the consideration of nonlinear core corrections. The features of either the ordinary Jahn-Teller conical intersections in X(3) (X=Li, Na, K, Cu, Ag, Au) trimers, or the elliptic Jahn-Teller conical intersections in NaH(2), have been well reproduced. In particular, anticipated results for the H-H(2) collision near the avoided crossing are obtained, showing appealing improvement over the first, real-time, TDDFT calculation. The other important type of intersections, Renner-Teller glancing intersection, has also been studied for several typical molecular systems (BH(2), AlH(2), CH(2)(+), SiH(2)(+)), giving results in reasonable agreement with the theoretical model. Despite these successes, it is found that for some systems, including both Jahn-Teller and Renner-Teller systems, the pseudopotential scheme might give inaccurate results for some NAC components on nonhydrogen atoms. By trying different construction schemes of pseudopotentials, e.g., using local pseudopotentials, the results of NACs are found scheme-dependent and show improvement for some cases. Since there is much freedom in constructing ab initio nonlocal pseudopotentials, our findings on TDDFT calculation of NACs in the pseudopotential scheme might be helpful to give clues for constructing more "realistic" pseudopotentials.

Implementation of a Density Functional Theory-Based Method for the Calculation of the Hyperfine A-tensor in Periodic Systems with the Use of Numerical and Slater Type Atomic Orbitals: Application to Paramagnetic Defects

The A-tensor parameterizes the "hyperfine" interaction of an "effective" electronic spin with the magnetic field due to the nuclear spin as monitored in an electron paramagnetic resonance (EPR) experiment. In this account, we describe an implementation for the calculation of the A-tensor in systems with translational invariance based on the Kohn-Sham form of density functional theory (KS DFT). The method is implemented in the periodic program BAND, where the Bloch states are expanded in the basis of numerical and Slater-type atomic orbitals (NAOs/STOs). This basis is well-suited for the accurate representation of the electron density near the nuclei, a prerequisite for the calculation of highly accurate hyperfine parameters. Our implementation does not rely on the frozen core approximation tacitly assumed in the pseudopotential schemes. The implementation is validated by performing calculations on the A-tensor for small atoms and molecules within the supercell approach as well as for paramagnetic defects in solids. In particular, we consider the A-tensor of "normal" and "anomalous" muonium defects in diamond and of the hydrogen cyanide anion radical HCN(-) in a KCl host crystal lattice.

Construction of Effective Model for Heavy Fermion Metal YbAl3with Hybridization Effect between Nearly Free Electron Bands and f-States and its Analysis

We construct an effective model of heavy fermion metal YbAl 3 . The nearly free electron method with the empty-core pseudo-potential scheme is employed for conduction electrons, which hybridize with localized 4f-electrons. The Hamiltonian includes only a few parameters estimated from the comparison with the results of the band calculations and experiments. We show that our model never yields a hybridization gap obtained by using the simple theoretical models, and clarify the origin of the mid-infrared peak structure observed in the optical conductivity.

Compton profiles and band structure calculations of IV–VI layered compounds GeS and GeSe

First ever isotropic experimental Compton profiles of GeS and GeSe are presented. Moreover, we present Compton profiles, energy bands and density of states (DOS) using Hartree–Fock, density functional and pseudopotential schemes. It is seen that the Hartree–Fock and density functional theories show a reasonable agreement with the experiment. The equal-valence-electron-density profiles show that GeS is more ionic than GeSe. We have also reported energy bands and DOS using full potential linearized augmented plane-wave method.

Theoretical investigations of structural, elastic, electronic and thermal properties of Damiaoite PtIn 2

In this paper, we report theoretical calculations based on ab initio total energy calculations using the full potential linearized augmented plane wave (FP-LAPW) method and the plane wave method with ultrasoft pseudo-potential (UPP) scheme in the frame of generalized gradient approximation (GGA). These calculations are the first step in the investigation of structural, electronic properties of Damiaoite PtIn 2. The thermodynamic properties will be calculated using the quasi-harmonic Debye model, in which the isothermal bulk modulus, thermal expansion coefficient, and specific heat for high pressures and temperatures are investigated. The major electronic trends show that PtIn 2 in Damiaoite structure is metallic with covalent bonding nature. The elastic properties calculations show that our system is mechanically stable but lower hard.

First-principles calculations of structure and high pressure phase transition in gallium nitride

The phase transitions of semiconductor GaN from the Wurtzite (WZ) structure and the zinc-blende (ZB) structure to the rocksalt (RS) structure are investigated by using the first-principles plane-wave pseudopotential density functional method combined with the ultrasoft pseudopotential scheme in the generalized gradient approximation (GGA) correction. It is found that the phase transitions from the WZ structure and the ZB structure to the RS structure occur at pressures of 46.1 GPa and 45.2 GPa, respectively. The lattice parameters, bulk moduli and their pressure derivatives of these structures of GaN are also calculated. Our results are consistent with available experimental and other theoretical results. The dependence of the normalized formula-unit volume V/V0 on pressure P is also successfully obtained.

Electronic, elastic and phonon properties of the rock-salt LaSb and YSb

The electronic structures and elastic constants of LaSb and YSb are calculated using an abinitio pseudopotential scheme within the generalized gradient approximation. In agreementwith experimental studies, it has been found that the upper valence bands in LaSb arecharacterized by Sb-5p and La-5d states. Our calculated elastic constants for LaSb arecompared with available experimental data. A linear-response approach to densityfunctional theory is used to derive phonon dispersion curves and the density of states forLaSb and YSb. Our phonon results for LaSb are in good agreement with experimentaldata. Frequency differences between phonon modes in these materials are discussed interms of differences involving total mass, reduced mass and the lattice constant.

Preparation and electrical properties of Ca 5Si 3 and Sr 5Si 3 powders

Single phase Ca 5Si 3 and Sr 5Si 3 powders were prepared, and their electric and thermoelectric properties were investigated. The Ca 5Si 3 and Sr 5Si 3 powders are synthesized by exposure of the Si powders to Ca and Sr fluxes, respectively. It is found that both silicides show a p-type conduction and semiconductor-like behavior. The electronic band structures of the silicides are calculated using the first-principles total-energy calculation program in pseudopotential schemes with plane-wave basis functions. The calculated result predicts the possibility of a semiconductor-like property with a sharp pseudogap at the Fermi level for Ca 5Si 3, as experimentally obtained. The silicide would be expected to be a new semiconductor-like conducting material.

Ab initiostructural and electronic investigation of magneticRNiSn(R=La,Ce,Pr,Nd)intermetallics and their hydrides

We investigated the electronic structure of the $R\mathrm{Ni}\mathrm{Sn}$ $(R=\mathrm{La},\mathrm{Ce},\mathrm{Pr},\mathrm{Nd})$ intermetallic compounds as well as their corresponding mono- and dihydrides. We computed the energy bands and the total and partial densities of states using a mixed approach; a pseudopotential scheme and ab initio full potential\char21{}all electron band-structure techniques. The crystal structure was investigated for each of the 12 compounds by performing full geometry optimization. We determine the magnetic properties of these compounds. We also analyze the details of the bonding in these systems, in particular, the effect of the hydrogen insertion in the parent intermetallic compounds. We found that, besides the nonmagnetic reference systems $\mathrm{La}\mathrm{Ni}\mathrm{Sn}{\mathrm{H}}_{x}$, the Pr- and Nd-derived systems are consistent with ${f}^{+2}$ and ${f}^{+3}$ electronic configurations. The case of the Ce compounds, as expected, showed a more complicated situation; the computed magnetic moment suggests an intermediate valence state for CeNiSnH. The decomposition enthalpies for the mono- and dihydrides have been estimated and compared with experimental values for similar compounds.

Phonon frequencies and related parameters in Ga xIn 1−xSb and InAs xP 1−x

For the anionic GaxIn 1− x Sb and cationic InAs x P 1− x mixed crystals, the longitudinal and transversal optical phonon frequencies, Fröhlich coupling parameter, Debye temperature of the longitudinal phonon frequency and effective polar field are obtained from band-structure calculations based on the pseudopotential scheme. The results regarding optical phonon frequencies of the hosts InP, InAs, InSb and GaSb crystals are compared with experiment and showed generally satisfactory agreement. For the remaining studied quantities, the present results are predictions.

First-principles dynamics treatment of light emission in collisions between alkali-metal atoms and noble-gas atoms at10keV

Collision-induced light emission during the interaction of an alkali-metal atom and a noble-gas atom is treated within a first-principles, or direct, dynamics approach that calculates a time-dependent electric dipole for the whole system, and spectral emission cross sections from its Fourier transform. These cross sections are very sensitive to excited diatomic potentials and a source of information on their shape. The coupling between electronic transitions and nuclear motions is treated with atomic pseudopotentials and an electronic density matrix coupled to trajectories for the nuclei. A recently implemented pseudopotential parametrization scheme is used here for the ground and excited states of the LiHe system, and to calculate state-to-state dipole moments. To verify the accuracy of our new parameters, we recalculate the integral cross sections for the LiHe system in the keV energy regime and obtain agreement with other results from theory and experiment. We further present results for the emission spectrum from $10\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ $\mathrm{Li}(2s)+\mathrm{He}$ collisions, and compare them to experimental values available in the region of light emitted at $300--900\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

Development of fully relativistic pseudopotentials and application to a thin magnetic film on the metal

We have developed a fully relativistic pseudopotential in the planewave method. The pseudopotential shows the sufficient transferability that the error is much smaller than the energy splitting of spin-orbit coupling. This pseudopotential scheme has been implemented for bulk systems, surfaces, and the thin magnetic film on the metal.

Alloy composition and optoelectronic properties of dilute GaSb 1−xN x by pseudo-potential calculations

We present a theoretical study of the electronic structure and optical properties of zinc-blende GaSb 1− x N x semiconducting dilute alloys for small amounts of N (0⩽ x⩽0.05). Our calculations are based on the pseudo-potential scheme within the virtual crystal approximation that takes into consideration the effects of compositional disorder. In agreement with previous studies, it is found that the introduction of only a few percent of N into GaSb reduces substantially the energy band gaps. It is shown that GaSb 1− x N x can be a direct or an indirect semiconductor depending on the N content. Negative band gaps in the material of interest are predicted to occur when the N concentration exceeds 0.026. The information derived from the present study shows that GaSb 1− x N x (0⩽ x⩽0.017) should enable access to wavelengths in the 2–4 μm range.

Theoretical and experimental study of CaWO4 and SrWO4 under pressure

Abstract In this paper, we combine a theoretical study of the structural phases of CaWO 4 and SrWO 4 under high pressure along with the results of angle-dispersive X-ray diffraction (ADXRD) and X-ray absorption near-edge structure (XANES) measurements of both tungstates up to approximately 20 GPa. The theoretical study was performed within the ab initio framework of the density functional theory (DFT) using a plane-wave basis set and the pseudopotential scheme, with the generalized gradient approximation (GGA) for the exchange and correlation contribution to the energy. Under normal conditions, CaWO 4 and SrWO 4 crystallize in the scheelite structure. Our results show that in a hydrostatic environment, both compounds undergo a scheelite-to-fergusonite phase transition with increasing pressure. We present a comparison of the evolution of the structural parameters, equation of state, and of the features of the transition, finding an overall excellent agreement between the experimental and theoretical results.