Search results for " orbita"
showing 10 items of 447 documents
Quantum Chemical Calculations Show that the Uranium Molecule U2 Has a Quintuple Bond.
2005
Covalent bonding is commonly described by Lewis's theory1, with an electron pair shared between two atoms constituting one full bond. Beginning with the valence bond description2 for the hydrogen molecule, quantum chemists have further explored the fundamental nature of the chemical bond for atoms throughout the periodic table, confirming that most molecules are indeed held together by one electron pair for each bond. But more complex binding may occur when large numbers of atomic orbitals can participate in bond formation. Such behaviour is common with transition metals. When involving heavy actinide elements, metal–metal bonds might prove particularly complicated. To date, evidence for ac…
Two- and three-state conical intersections in the electron capture dissociation of disulfides: The importance of multireference calculations
2011
The SS bond cleavage produced upon electron attachment to disulfides was generally assumed to be an adiabatic process because the added electron occupies the σ*(SS) antibonding orbital. This is clearly the case in the parent HSSH compound, but not necessarily in XSSX′ derivatives, where the substituents X and X′ are different. Through the use of MS-CASPT2 calculations, we have shown that the dissociation of the SS two-center-three-electron bond in these asymmetric XSSX′ compounds requires the interaction of at least two states, in order to localize the extra electron in one of the fragments upon dissociation. This is actually the case for the CH3SSNH2 derivative, where the most favorable di…
Stability of Borane−Adduct Complexes: A G-2 Molecular Orbital Study
1997
Complexation energies of H3BXHn and [H3BXHn-1]- complexes (X = N, O, F, P, S, and Cl) (n = 3, 2, 1) have been computed at the G-2 level of theory. The formation of H3BXH3 (X = N, P) is found to be more favored than the formations of H3BXH2 (X = O, S) and H3BXH (X = F, Cl). The qualitative features of the molecular orbital interaction (the correlation diagrams) of H3BNH3 (C3v symmetry group), H3BOH2 (Cs symmetry group), and H3BFH (Cs symmetry group) complexes are presented. These diagrams show that the σ character of the B−X bond decreases and the π character increases when the electronegativity of X increases and indicate that the B−X bond cannot be treated only in terms of the simplest mod…
Nuclear magnetic shielding constants in the CC2 model
1997
Abstract Test calculations of nuclear magnetic shielding constants in the CC2 model are performed using the gauge-including atomic orbital approach. Absolute shielding constants are reported for reprsentative first-row hydrides, a few multiply bonded molecules and some challenging cases. The performance of CC2 is analyzed by comparison with experimental data and results from calculations employing more sophisticated treatments of electron correlation. In most cases, CC2 shieldings and chemical shifts are close to those obtained at second-order perturbation theory, despite the fact that the CC2 model includes an approximate treatment of orbital relaxation effects at the correlated level.
Ab Initio Methods for Excited States
2005
This chapter focuses mainly on the performance of ab initio methods for the description of spectroscopic molecular properties of compounds. Most of the quantum-chemical methods developed up to date are based on the concept of the one-electron wave function. The electronic states of a system with N electrons are described by a double expansion. Molecular orbitals (MOs) are one-electron wave functions expressed as linear combinations of a known one-electron basis set (K) and the N electron wave function is formulated in a many-electron basis set formed by determinants (or linear combination of them to form spin-adapted wave functions), built as normalized antisymmetric products of MOs. Accord…
Ab initio molecular orbital study of SenS4−nN4 (n = 0−4)
1995
Abstract We report an ab initio study of Se n S 4− n N 4 ( n = 0−4). The full geometry optimization for each molecule was performed at the Hartree-Fock level of theory involving the MIDI-4 ∗ basis sets for atomic orbitals. The correction for electron correlation was carried out for optimized geometries by utilizing the second-order Moller-Plesset (MP2) perturbation theory. The fundamental vibrations calculated for all molecular species verified that all molecules lie at the local minima. All molecules showed cage structures similar to those observed experimentally for S 4 N 4 and Se 4 N 4 . The calculated bond parameters of S 4 N 4 and Se 4 N 4 were in good agreement with the experimental v…
Quantitative prediction of gas-phase F19 nuclear magnetic shielding constants
2008
Benchmark calculations of (19)F nuclear magnetic shielding constants are presented for a set of 28 molecules. Near-quantitative accuracy (ca. 2 ppm deviation from experiment) is achieved if (1) electron correlation is adequately treated by employing the coupled-cluster singles and doubles (CCSD) model augmented by a perturbative correction for triple excitations [CCSD(T)], (2) large (uncontracted) basis sets are used, (3) gauge-including atomic orbitals are used to ensure gauge-origin independence, (4) calculations are performed at accurate equilibrium geometries [obtained from CCSD(T)/cc-pVTZ calculations correlating all electrons], and (5) vibrational averaging and temperature corrections…
A two-scale approach to electron correlation in multiconfigurational perturbation theory.
2014
We present a new approach for the calculation of dynamic electron correlation effects in large molecular systems using multiconfigurational second-order perturbation theory (CASPT2). The method is restricted to cases where partitioning of the molecular system into an active site and an environment is meaningful. Only dynamic correlation effects derived from orbitals extending over the active site are included at the CASPT2 level of theory, whereas the correlation effects of the environment are retrieved at lower computational costs. For sufficiently large systems, the small errors introduced by this approximation are contrasted by the substantial savings in both storage and computational de…
First Principles Calculations of Atomic and Electronic Structure of Ti3+Al- and Ti2+Al-Doped YAlO3
2021
M.G.B. appreciates support from the Chongqing Recruitment Program for 100 Overseas Innovative Talents (grant no. 2015013), the Program for the Foreign Experts (grant no. W2017011), Wenfeng High-end Talents Project (grant no. W2016-01) offered by the Chongqing University of Posts and Telecommunications (CQUPT), Estonian Research Council grant PUT PRG111, European Regional Development Fund (TK141), and NCN project 2018/31/B/ST4/00924. This study was supported by a grant from Latvian Research Council No. LZP-2018/1-0214 (for AIP). Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Program H202…
Towards an accurate molecular orbital theory for excited states : Ethene, butadiene, and hexatriene
1993
A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans‐1,3‐butadiene, and trans‐trans‐1,3,5‐hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic ele…