Search results for " quantum chemistry"
showing 10 items of 549 documents
Full configuration-interaction and coupled-cluster calculations of the indirect spin–spin coupling constant of BH
2003
Abstract Full configuration-interaction calculations of the indirect spin–spin coupling constant of the BH molecule have been carried out in order to investigate the performance of various coupled-cluster (CC) methods in the treatment of electron-correlation effects, while the corresponding basis set convergence is analyzed in CC singles and doubles calculations. Assuming additivity of correlation and basis set effects, a theoretical estimate of 50.67 Hz is obtained for the 11 B 1 H spin–spin coupling constant.
The route to high accuracy in ab initio calculations of Cu quadrupole-coupling constants.
2012
We report nonrelativistic and scalar-relativistic coupled-cluster calculations of the copper quadrupole-coupling constants for eleven small copper-containing compounds. It is shown to be necessary to treat both electron-correlation and scalar-relativistic effects on the same footing even for a qualitatively correct description, because both effects are significant and are strongly coupled in the case of Cu electric-field gradients. We show that the three scalar-relativistic schemes employed in the present study--the leading order of direct perturbation theory, the spin-free exact two-component theory in its one-electron variant, and the spin-free Dirac-Coulomb approach--provide accurate tre…
Relativistic DFT Calculation of (119)Sn Chemical Shifts and Coupling Constants in Tin Compounds.
2006
The nuclear shielding and spin-spin coupling constants of (119)Sn in stannane, tetramethylstannane, methyltin halides Me4-nSnXn (X = Cl, Br, I; n = 1-3), tin halides, and some stannyl cations have been investigated computationally by DFT methods and Slater all-electron basis sets, including relativistic effects by means of the zeroth order regular approximation (ZORA) method up to spin-orbit coupling. Calculated (119)Sn chemical shifts generally correlate well with experimental values, except when several heavy halogen atoms, especially iodine, are bound to tin. In such cases, calculated chemical shifts are almost constant at the scalar (spin-free) ZORA level; only at the spin-orbit level i…
Spin-orbit, radial, and angular coupling effects in the NaRb excited states
2009
Spin-orbit, radial, and angular nonadiabatic matrix elements between the lowest excited states of NaRb are evaluated by quasi-relativistic ab initio methods, and the results accompanied by potential curves, permanent and transition moments are compared with experimental data and preceding calculations.
Comparative ab initio calculations of SrTiO3 and CaTiO3 polar (111) surfaces
2012
We present the results of calculations of SrTiO3 and CaTiO3 polar (111) surface relaxations, rumplings, energetics, optical band gaps, and charge distributions using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We have calculated the surface relaxation of the two possible terminations (Ti and SrO3 or CaO3) of the SrTiO3 and CaTiO3 (111) surfaces. According to our calculations, atoms of the first surface layer relax inwards for Ti-, SrO3-, and CaO3-terminated (111) surfaces of both materials. The only exception is outward relaxation of the SrO3-terminated SrTiO3 (111) surface upper layer Sr atom. For both Ti-terminated SrTiO3 and CaTiO3 (111) surfaces our …
Ab initiocalculations of theHcenters in MgF2crystals
2012
MgF2 with rutile structure is important wide-gap optical material with numerous applications. We present and discuss the results of calculations for basic hole defects - interstitial F atoms (called also the colour H centres). This study is based on the large scale ab initio DFT calculations using hybrid B3PW exchange-correlation functional as implemented into CRYSTAL computer code. The electronic structure, atomic geometry, charge density distribution are calculated and discussed.
Ab initio structural and electronic band-structure study of MgSe
2015
We present a careful and detailed ab initio study of the crystal structure and electronic band structure of different crystalline phases of MgSe. Calculations were performed using the full-potential linear augmented plane wave method. To determine the crystal phase of the ground state of MgSe, we computed the total energy as a function of volume for the rock-salt, zinc-blende, wurtzite, and NiAs phases. From the optimized volume, and by using the Birch–Murnaghan equation of state the lattice parameters , the bulk moduli, and its first pressure derivative () for the different phases of MgSe were found. In our calculations, we have used the local density approach for the exchange–correlation …
Ab initio calculations of SrTiO3, BaTiO3, PbTiO3, CaTiO3, SrZrO3, PbZrO3 and BaZrO3 (001), (011) and (111) surfaces as well as F centers, polarons, K…
2014
In this paper, the review of recent results of calculations of surface relaxations, energetics, and bonding properties for ABO 3 perovskite (001), (011) and (111) surfaces using mostly a hybrid description of exchange and correlation is presented. Both AO and BO 2-terminations of the nonpolar (001) surface and A , BO , and O terminations of the polar (011) surface, as well as B and AO 3-terminations of the polar (111) surface were considered. On the AO -terminated (001) surface, all upper-layer A atoms relax inwards, while all second layer atoms relax outwards. For the BO 2-terminated (001) surface, in most cases, the largest relaxations are on the second-layer metal atoms. For almost all …
The Effect of Oxygen Vacancies on the Atomic and Electronic Structure of Cubic ABO3Perovskite Bulk and the (001) Surface:Abinitio Calculations
2009
We employed the hybrid DFT-LCAO and GGA-PW approaches as implemented in the CRYSTAL and VASP codes, respectively, for large supercell calculations of neutral O vacancies with trapped electrons (known as F centers) in the bulk and on the (001) surface of three cubic perovskite crystals (SrTiO 3 , PbTiO 3 , and PbZrO 3 ). The local lattice relaxation, charge redistribution, and positions of defect energy levels within the band gap are compared for three perovskites under study. We demonstrate how the difference in chemical composition of host materials leads to quite different defect properties.
Ab initiocalculations of BaTiO3(111) surfaces
2013
The article presents the results of calculations of polar (111) surface relaxations and energetics using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. Using a hybrid B3LYP approach, the surface relaxation has been calculated for the two possible Ti and BaO3 BaTiO3 (111) surface terminations. For both Ti- and BaO3-terminated BaTiO3 (111) surfaces, the upper layer atoms relax inward. The second layer atoms, with the sole exception of Ti-terminated BaTiO3 (111) surface Ba atom, relax outward. The calculated surface relaxation energy for Ti-terminated BaTiO3 (111) surface is more than two times larger than the surface relaxation energy for BaO3-terminated BaTi…