Search results for "QUANTUM CHEMISTRY"

Bader’s topological analysis of the electron density in the pressure-induced phase transitions/amorphization in α-quartz from the catastrophe theory …

2013

In this work, the Bader's topological analysis of the electron density, coupled with Thom's catastrophe theory, was used to characterize the pressure-induced transformations in α-quartz. In particular, ab initio calculations of the α-quartz structures in the range 0-105 Gpa have been performed at the HF/DFT exchange-correlation terms level, using Hamiltonians based on a WC1LYP hybrid scheme. The electron densities calculated throughout the ab initio wave functions have been analysed by means of the Bader's theory, seeking for some catastrophic mechanism in the sense of Thom's theory. The analysis mainly showed that there is a typical fold catastrophe feature involving an O-O interaction at …

Ab initio calculations of the atomic and electronic structure of BaZrO3 (111) surfaces

2013

Abstract The paper presents and discusses the results of calculations of surface relaxations and energetics for the polar (111) surface of BaZrO 3 using a hybrid B3LYP description of exchange and correlation. On the (111) surface, both Zr- and BaO 3 -terminations were analyzed. For both Zr and BaO 3 -terminated BaZrO 3 (111) surface upper layer atoms, with the sole exception of BaO 3 -terminated surface Ba atoms, relax inwards. The Zr-terminated BaZrO 3 (111) surface second layer Ba atoms exhibit the strongest relaxation between all Zr and BaO 3 -terminated BaZrO 3 (111) surface atoms. The calculated surface relaxation energy for Zr-terminated BaZrO 3 (111) surface is almost fifteen times l…

First principles calculations of SrZrO3 bulk and ZrO2-terminated (001) surface F centers

2016

Abstract Using a supercell model and B3PW hybrid exchange-correlation functional in the framework of the density functional theory (DFT), as it is implemented in the CRYSTAL computer code, we performed ab initio calculations for the F -center located in the SrZrO 3 bulk and on the ZrO 2 -terminated (001) surface. According to the results of performed relaxation of atoms around the defect, two nearest Zr and four Sr atoms are repulsed, but all oxygen atoms are attracted towards both, the bulk and (001) surface F -center. The displacement magnitudes of atoms surrounding the bulk F -center are smaller than around the (001) surface F -center. The B3PW calculated SrZrO 3 bulk optical band gap (5…

Electron delocalization in mixed-valence Keggin polyoxometalates. Ab initio calculation of the local effective transfer integrals and its consequence…

2002

We present a quantitative evaluation of the influence of the electron transfer on the magnetic properties of mixed-valence polyoxometalates reduced by two electrons. For that purpose, we extract from valence-spectroscopy ab initio calculations on embedded fragments the value of the transfer integrals between W nearest-neighbor atoms in a mixed-valence alphaPW(12)O(40) polyoxowolframate Keggin anion. In contradiction with what is usually assumed, we show that the electron transfer between edge-sharing and corner-sharing WO(6) octahedra have very close values. Considering fragments of various ranges, we analyze the accuracy of calculations on fragments based on only two WO(5) pyramids which s…

From CCSD(T)/aug-cc-pVTZ-J to CCSD(T) complete basis set limit isotropic nuclear magnetic shieldings via affordable DFT/CBS calculations

2011

It is shown that a linear correlation exists between nuclear shielding constants for nine small inorganic and organic molecules (N2, CO, CO2, NH3, CH4, C2H2, C2H4, C2H6 and C6H6) calculated with 47 methods (42 DFT methods, RHF, MP2, SOPPA, SOPPA(CCSD), CCSD(T)) and the aug-cc-pVTZ-J basis set and corresponding complete basis set results, estimated from calculations with the family of polarization-consistent pcS-n basis sets. This implies that the remaining basis set error of the aug-cc-pVTZ-J basis set is very similar in DFT and CCSD(T) calculations. As the aug-cc-pVTZ-J basis set is significantly smaller, CCSD(T)/aug-cc-pVTZ-J calculations allow in combination with affordable DFT/pcS-n com…

Explicitly Correlated Electrons in Molecules

2011

Dependence of the Substituent Effect on Solvent Properties

2018

The influence of a solvent on the substituent effect (SE) in 1,4-disubstituted derivatives of benzene (BEN), cyclohexa-1,3-diene (CHD), and bicyclo[2.2.2]octane (BCO) is studied by the use of polarizable continuum model method. In all X–R–Y systems for the functional group Y (NO2, COOH, OH, and NH2), the following substituents X have been chosen: NO2, CHO, H, OH, and NH2. The substituent effect is characterized by the charge of the substituent active region (cSAR(X)), substituent effect stabilization energy (SESE), and substituent constants σ or F descriptors, the functional groups by cSAR(Y), whereas π-electron delocalization of transmitting moieties (BEN and CHD) is characterized by a geo…

How far the substituent effects in disubstituted cyclohexa-1,3-diene derivatives differ from those in bicyclo[2.2.2]octane and benzene?

2018

Substituents effects in cyclic diene derivatives are studied using quantum chemical modeling and compared to the corresponding effects in aromatic (benzene) and fully saturated (bicyclo[2.2.2]octane) compounds. In particular, electronic properties of the fixed group Y in a series of 3- and 4-X-substituted cyclohexa-1,3-diene-Y derivatives (where Y = NO2, COOH, COO− OH, O−, NH2, and X = NMe2, NH2, OH, OMe, Me, H, F, Cl, CF3, CN, CHO, COMe, CONH2, COOH, NO2, NO) are examined using the B3LYP/6-311++G(d,p) method. For this purpose, quantum chemistry models of the substituent effect: cSAR (charge of the substituent active region) and SESE (substituent effect stabilization energy) as well as trad…

Electronic aspects of the hydride transfer mechanism. Ab initio analytical gradient studies of the cyclopropenyl‐cation/lithium hydride model reactan…

1985

The electronic mechanisms of a model hydride transfer reaction are theoretically studied with ab inito RHF and UHF SCF MO procedures at the 4‐31G basis set level and analytical gradient methods. The model system describes the reduction of cyclopropenyl cation to cyclopropene by the oxidation of lithium hydride to lithium cation. The molecular fragments corresponding to the asymptotic reactive channels characterizing the stepwise mechanisms currently discussed in the literature have been characterized. The binding energy between the fragments is estimated within a simple electrostatic approximate scheme. The results show that a hydride‐ion mechanism is a likely pathway for this particular sy…

Quantum chemical modelling of electron polarons and excitons in ABO3perovskites

2000

Quantum chemical calculations using the intermediate neglect of the differential overlap (INDO) method, combined with the large unit cell periodic model argue for an existence of the self-trapped electrons in KNbO3 and KTaO3 perovskite crystals. An electron in the ground state occupies predominantly t2g orbital of a Nb4+ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn-Teller modes where four nearest equatorial O atoms are displaced outwards and two oxygens shift inwards along the z axis. Triplet exciton is shown to be in a good approximation of a pair of nearest Jahn-Teller electron and hole polarons (a bipolaron) which is very likely responsible for the `gr…