Search results for "ab initio"
showing 10 items of 990 documents
Ab initio calculations for the polar (0 0 1) surfaces of YAlO3
2018
Abstract The results of ab initio calculations of polar YAlO3 (0 0 1) surfaces by means of a hybrid B3LYP exchange-correlation functional as it is implemented in the CRYSTAL computer code are presented. Both polar YO and AlO2-terminations of the cubic YAlO3 (0 0 1) surface were considered. We performed relaxation of atoms on the upper three layers of both YO and AlO2-terminated YAlO3 (0 0 1) surfaces using in our calculations slabs containing 22 and 23 atoms as well as 9 layers, respectively. We predict a significant increase of the Al-O chemical bond covalency on the AlO2-terminated YAlO3 (0 0 1) surface with respect to the YAlO3 bulk. Our calculated YO and AlO2-terminated YAlO3 polar (0 0…
The electronic and atomic structure of SrTiO3, BaTiO3, and PbTiO3(001) surfaces: Ab initio DFT/HF hybrid calculations
2005
In our first-principles study, the electronic properties of the (001) surfaces of three key perovskite crystals, namely SrTiO"3 (STO), BaTiO"3 (BTO), and PbTiO"3 (PTO), have been calculated by means of the density functional theory (DFT) using the exchange-correlation functional containing ''hybrid'' of the non-local Hartree-Fock (HF) exchange, DFT exchange, and Generalized Gradient Approximation (GGA) correlation functionals, commonly known as B3PW. Such a technique allows us to get the optical bulk band gap very close to experiment unlike previous calculations of perovskites. Special attention is paid to careful calculations of the surface rumpling and change of the distances between thre…
Study of High-Temperature Behaviour of ZnO by Ab Initio Molecular Dynamics Simulations and X-ray Absorption Spectroscopy
2021
Wurtzite-type zinc oxide (w-ZnO) is a widely used material with a pronounced structural anisotropy along the c axis, which affects its lattice dynamics and represents a difficulty for its accurate description using classical models of interatomic interactions. In this study, ab initio molecular dynamics (AIMD) was employed to simulate a bulk w-ZnO phase in the NpT ensemble in the high-temperature range from 300 K to 1200 K. The results of the simulations were validated by comparison with the experimental Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra and known diffraction data. AIMD NpT simulations reproduced well the thermal expansion of the lattice, and the pronounced …
Psi4: an open-source ab initio electronic structure program
2011
The Psi4 program is a new approach to modern quantum chemistry, encompassing Hartree–Fock and density-functional theory to configuration interaction and coupled cluster. The program is written entirely in C++ and relies on a new infrastructure that has been designed to permit high-efficiency computations of both standard and emerging electronic structure methods on conventional and high-performance parallel computer architectures. Psi4 offers flexible user input built on the Python scripting language that enables both new and experienced users to make full use of the program's capabilities, and even to implement new functionality with moderate effort. To maximize its impact and usefulness, …
Indium-Gallium Segregation inCuInxGa1−xSe2: AnAb Initio–Based Monte Carlo Study
2010
Thin-film solar cells with ${\mathrm{CuIn}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Se}}_{2}$ (CIGS) absorber are still far below their efficiency limit, although lab cells already reach 20.1%. One important aspect is the homogeneity of the alloy. Large-scale simulations combining Monte Carlo and density functional calculations show that two phases coexist in thermal equilibrium below room temperature. Only at higher temperatures, CIGS becomes more and more a homogeneous alloy. A larger degree of inhomogeneity for Ga-rich CIGS persists over a wide temperature range, which contributes to the observed low efficiency of Ga-rich CIGS solar cells.
Comparison of Permeation Measurements and Hybrid Density Functional Calculations on Oxygen Vacancy Transport in Complex Perovskite Oxides
2014
The oxygen vacancy (VO••) formation enthalpy (ΔHF) was determined from the effective activation energies obtained in the permeation measurements on a series of solid solutions La0.98–xSrxCo0.2Fe0.8O3−δ (x = 0.125–0.8) and compared with the results of ab initio calculations at finite temperatures based on the hybrid (PBE0) and GGA+U calculations, as well as previous thermogravimetric experiments. The Mulliken atomic charges, magnetic moments, and the Fe-VO•• and Sr-VO•• distances are analyzed. The strong dependence of formation enthalpy of VO•• on the Fe oxidation state due to variation in Sr and VO•• concentrations is discussed.
Through-space spin-spin coupling in acetylenic systems. Ab initio and DFT calculations
2003
Abstract: We have investigated, by means of ab initio and DFT calculations, the magnitude of through-space spin-spin couplings ( J CH and J HH ) in CH/π bonded van der Waals dimers involving acetylene, and in a structurally related covalent compound (4-ethynylphenanthrene). Within regions where the interaction is stabilizing J HH couplings are very small (< 0.1 Hz) for all complexes. In the acetylene-methane complex J CH is also very small, whereas in the acetylene-benzene complex and the acetylene dimer it shows a relatively large dependence on the tilt angle from the T-shaped arrangement, for which the smallest values are calculated, to a parallel slipped arrangement where J CH is ca. 0.5…
Molecular orbital studies on brominated diphenyl ethers. Part II—reactivity and quantitative structure–activity (property) relationships
2005
Polybrominated diphenyl ethers (PBDEs) are widely used as flame retardants and are increasingly turning up in the environment. Their structural similarities to polychlorinated biphenyls and thyroid hormones suggest they may be a risk to human health. The present study examines the reactivity of brominated diphenyl ethers (BDEs) on the basis of the electronic structures as calculated by semiempirical AM1 self-consistent field molecular orbital (SCF-MO) method. Frontier orbital energies were used to elucidate the reactivity of BDEs in electrophilic, nucleophilic and photolytic reactions. From an examination of the frontier electron densities, the regioselectivity, or orientation, of metabolic…
First Principles Calculations of Atomic and Electronic Structure of TiAl3+- and TiAl2+-Doped YAlO3
2021
In this paper, the density functional theory accompanied with linear combination of atomic orbitals (LCAO) method is applied to study the atomic and electronic structure of the Ti3+ and Ti2+ ions substituted for the host Al atom in orthorhombic Pbnm bulk YAlO3 crystals. The disordered crystalline structure of YAlO3 was modelled in a large supercell containing 160 atoms, allowing simulation of a substitutional dopant with a concentration of about 3%. In the case of the Ti2+-doped YAlO3, compensated F-center (oxygen vacancy with two trapped electrons) is inserted close to the Ti to make the unit cell neutral. Changes of the interatomic distances and angles between the chemical bonds in the de…