Effects of nonlocal correlation functionals on electron-phonon interactions in NbC

As a test for the applicability of the density functional theory (DFT) to hydrogen-bonded systems, various local and nonlocal exchange correlation functionals have been used to calculate the equilibrium structure and the harmonic vibrational frequencies of the pyridine−water complex. We also performed restricted Hartree−Fock (RHF) and Moller−Plesset (MP2) calculations for comparison. Three different basis sets (6-31G**, 6-31++G**, and 6-311++G(2d, 2p) 6d)) were used in the study. The best agreement between the DFT computed and the experimental rotational constants for monomeric pyridine were obtained with the B3-PW91 and B3-LYP functionals. For the vibrational frequencies of the H-bonded complex, the experimental data from matrix FT-IR spectrometry as well as the results obtained from the ab initio methods were best reproduced with the DFT method with the nonlocal exchange and correlation functionals. The hybrid approaches, which mix HF and Slater exchange, are particularly effective in these types of calculations.

Equilibrium geometries, bond dissociation energies, dipole moments, harmonic vibrational frequencies, and infrared intensities were calculated for a set of ten neutral nitrogen oxides (NO, NO2, NO3, N2O, sym N2O2, asym N2O3, sym N2O3, sym N2O4, asym N2O4, and N2O5) by applying one local and two gradient-corrected nonlocal functionals in a Gaussian-type-orbital density functional method. Comparison with available experimental data shows that, except for the bond dissociation energies, the local functional gives very accurate molecular properties. Nonlocal functionals considerably improve the bond dissociation energies, but the results still overestimate the experimental values by about 10 kcal/mol on average. For the other properties, the results obtained with nonlocal functionals are not necessarily superior to those calculated with the local functional. The properties of two molecules (sym N2O3 and asym N2O4) are predicted for the first time and several reassignments are proposed in the vibrational spectra of di-nitrogen oxides.

The O2 3−-Y3+ center in fluorite-type structures (CaF2 and SrF2) has been investigated at the density functional theory (DFT) level using the CRYSTAL06 code. Our calculations were performed by means of the hybrid B3PW method in which the Hartree–Fock exchange is mixed with the DFT exchange functional, using Becke’s three parameter method, combined with the non-local correlation functionals by Perdew and Wang. Our calculations confirm the stability and the molecular character of the O2 3−-Y3+ center. The unpaired electron is shown to be almost exclusively localized on and equally distributed between the two oxygen atoms that are separated by a bond distance of 2.47 A in CaF2 and 2.57 A in SrF2. The calculated 17O and 19F hyperfine constants for of the O2 3−-Y3+ center are in good agreement with their corresponding experimental values reported by previous electron paramagnetic resonance and electron nuclear double resonance studies, while discrepancies are notable for the 89Y hyperfine constants.

First-principles study of electronic structure, phonons and electron-phonon interaction in hexagonal PdTe

Density functional theory calculations with non-local correlation functionals, properly accounting for dispersion forces, predict the presence of two minima in the interaction energy between h-BN and Ni(111). These can be described as a physisorbed state with no corrugation of the h-BN structure, and a chemisorbed state exhibiting noticeable corrugation and a shorter distance of h-BN to the metallic support. The latter corresponds indeed to the one reported in most experiments. The relative stability of the two minima depends on the specific density functional employed: of those investigated here only optB86b-vdW yields the correct order of stability. We also demonstrate that the effect of the metal support on the Raman frequency of the chemisorbed boron nitride monolayer cannot be reduced to the associated strain. This is important because the Raman frequency has been proposed as a signature to identify h-BN monolayers from multilayered samples. Our analysis shows that such signatures would be strongly dependent on the nature of the interaction between the support and h-BN.

Ground-state electronic structure and optical properties of distorted perovskite DyMnO3 are investigated in the frame of density functional theory within generalized gradient approximation (GGA). Optimized lattice constants are reasonable in agreement with experimental data. Electronic band structure, density of states and partial density of states of elements are obtained. It shows that DyMnO3 exhibit an indirect band gap of 0.91 eV. Dielectric function, optical reflectivity, refractive index, extinction coefficient, electron energy loss, and absorption coefficient are calculated and analyzed for radiation up to 35 eV.

Preserving the beneficial properties of the second-order screened exchange (SOSEX) method, such as its freedom from one-body self-correlation error, and its seamless treatment of long-range dispersion interaction, we construct new nonlocal functionals by down-scaling the higher-order terms in the SOSEX series to reduce the many-body self-correlation error in molecular systems. Our down-scaled SOSEX (dsSOSEX) and scaled equation SOSEX (seSOSEX) approaches deliver considerably more accurate noncovalent interaction energies, reaction energies, and barrier heights than the original SOSEX method. The dsSOSEX approach improves on the description of medium- and long-range correlation, for example, in isogyric processes, while the seSOSEX approach improves on the description of processes with short- and medium-range rearrangement in the electronic structure, such as atomization. The He2+ potential energy curve shows that the many-body self-correlation error is slightly smaller in the dsSOSEX method than in the seSOSEX approach. Furthermore, the H2 potential energy curve reveals that the static correlation error can also be reduced at medium range by the dsSOSEX and seSOSEX schemes. As our dsSOSEX method provides a better description of medium- and long-range correlation, it generally leads to more accurate energetics than the seSOSEX method; hence it should be preferred over the latter. We compare our results to other empirical and nonempirical direct random phase approximation (dRPA) and beyond-dRPA methods and also discuss the possibilities for further development.

Structure and electronic properties of LnScO3 compounds: A GGA + U calculation

The semilocal form of an exchange hole is highly useful in developing non-local range-separated hybrid density functionals for finite and extended systems. The way to construct the conventional exact exchange hole model is based on either the Taylor series expansion or the reverse engineering technique from the corresponding exchange energy functional. Although the latter technique is quite popular in the context of generalized gradient approximation (GGA) functionals, the same for the meta-GGA functionals is not so much explored. Thus, in this study, we propose a reverse-engineered semilocal exchange hole of a meta-GGA functional, which only depends on the meta-GGA ingredient α (also known as the Pauli kinetic energy enhancement factor). The model is subsequently used to design the short-range-separated meta-GGA hybrid density functional. We show that the present method can be successfully applied for several challenging problems in the context of solids, especially for which the GGA based hybrid fails drastically. This assessment proves that the present functional is quite useful for materials sciences. Finally, we also use this method for several molecular test cases, where the results are also as comparative as its base semilocal functional.

We present results of electronic structure calculations of the bulk properties of the Weyl semimetal TaAs. The emergence of Weyl (massless) fermions in TaAs, due to its electronic band structure, is indicative of a new state of matter in the condensed phase that is of great interest for fundamental physics and possibly new applications. Many of the physical properties of the material, however, are unknown. We have calculated the structural parameters, dielectric function, elastic constants, phonon dispersion, electronic band structure, and Born effective charges using density functional theory within the generalized gradient approximation, including spin-orbit coupling where necessary. Our results provide essential information on the material; and our calculations agree well with the relatively small number of experimental data available. Moreover, we have determined the relative stability of the ground state body-centered tetragonal phase with respect to other common binary phases as a function of pressure at the athermal limit, predicting a transition to the CsCl cubic structure at 23.3 GPa. Finally, we have determined the band structure using an unbiased hybrid density functional that includes 25% exact exchange, in order to refine the previously determined positions in k space of the Weyl points.

Using first‐principle calculations, a new compound semiconductor based on titanium (Ti) is predicted. It has been observed that Ti when alloyed with germanium (Ge) can crystallize in the zincblende symmetry, and exhibits semiconducting nature with a narrow bandgap. To test the robustness of the semiconducting nature, the structural properties and electronic band structure have been modeled using three different exchange–correlation functionals, namely, local density approximation, generalized gradient approximation, and Perdew‐Burke‐Ernzerhof for solids. With a nominal composition of 1:1, TiGe exhibits a direct bandgap of 0.58 eV at the “X” point of the Brillouin zone. From lattice dynamics, the structural stability of the compound has been established. The new semiconductor is characterized in detail for its electronic band structure, carrier effective mass, charge density distribution, and linear optical properties. Zb‐TiGe exhibits large effective mass for conduction band holes and tentatively becomes a prototype system to study heavy holes resulting in carrier condensation at the bottom of the conduction band. The linear optical properties of the TiGe are found to be suitable for photosensitive devices in the infrared region. In addition, TiGe may find application in deep‐UV and far‐UV regions because of the surface plasmon resonance at 6.4 eV.

Electronic band structures and spin-charge densities of edge-functionalized silicene nanoribbons as promising candidates for either optoelectronics or spintronics applications

Density functional study of the H 3NCl 2 system—the importance of Hartree-Fock exchange in density functional methods

Density functional theory (DFT) has emerged as one of the most versatile and lucrative approaches in electronic structure calculations of many-electron systems in past four decades. Here we give an account of the development of a variational DFT method for atoms and molecules completely in a Cartesian grid. The non-relativistic Kohn–Sham equation is solved by using an LCAO-MO ansatz. Atom-centered localized basis set, electron density, molecular orbitals, two-body potentials are directly constructed on the grid. We adopt a Fourier convolution method for classical Coulomb potentials by making an Ewald-type decomposition technique in terms of short- and long-range interactions. It produces quite accurate and competitive results for various properties of interest, such as component energy, total energy, ionization energy, potential energy curve, atomization energy, etc. Both local and non-local functionals are employed for pseudopotential as well as full calculations. While most results are offered in a uniform grid, initial exploratory attempts are made in a non-uniform grid, which can significantly reduce the computational overhead. This offers a practical, viable alternative to atom-centered grid-based implementations, currently exploited by the majority of programs available world-wide.

The electronic band structures, densities of states (DOSs), and projected densities of states (PDOSs) of the wurtzite In1−xGaxN with x = 0, 0.0625, 0.125 are studied using the generalized-gradient approximation (GGA) and GGA + U in density functional theory. Our calculations suggest that in the case of wurtzite InN it is important to apply an on-site Hubbard correction to both the d states of indium and the p states of nitrogen in order to recover the correct energy level symmetry and obtain a reliable description of the InN band structure. The method is used to study the electronic properties of the wurtzite In1−xGaxN. The conduction band minimum (CBM) energy increases, while the valence band maximum (VBM) energy decreases with the increase of the gallium concentration. The effect leads to broadening the band gap (BG) and the valence band width (VBW). Furthermore, the compressive strain in the crystal can cause the BG and the VBW to increase with the increase of gallium concentrations.