Ab initio calculations on low-lying electronic states of PdH

Potential energy curves for the low-lying electronic states of PdH have been calculated using the MRCI method with scalar relativistic and spin-orbit corrections, and all electronic states correlating to the 4d¹⁰ (¹S), 4d⁹ 5s¹ (³D), 4d⁹ 5s¹ (¹D) and 4d⁸ 5s² (³F) states of Pd were included. Potential energy curves for the individual Ω states have been obtained, and the experimentally observed spectra of both PdH and PdD isotopologues have been assigned appropriately based on the ab initio results. Einstein A coefficients were calculated for other possible transitions from the low-lying electronic states to the X²Σ⁺ ground state. Diagonal and off-diagonal matrix elements of the spin-orbit Hamiltonian were calculated for all vibrational levels of the X²Σ⁺, 1²Δ, 1²Π, 2²Σ⁺ and 3²Σ⁺ states, and it was found from the eigenvectors that the vibrational wavefunctions of the 1²Δ_3/2 and 1²Π_3/2 states are mixed significantly in both PdH and PdD isotopologues.     

Ab initio study of low-lying electronic states of the PF2 radical

The equilibrium geometries, excitation energies, force constants, and vibrational frequencies of the low-lying electronic states X²B₁, ²A₁, ²B₂, and ²A₂ of the PF₂ radical have been calculated at the MRSDCI level with a double zeta plus polarization basis set. Our calculated geometry, force constants, and vibrational frequencies for the X²B₁ state are in good agreement with experimental data. The electronic transition moments, oscillator strengths for the ²A₁ → X²B₁ and ²A₂ → X²B₁ transitions, and radiative lifetimes for the ²A₁ and ²A₂ states are calculated based on the MRSDCI wave functions. © 1994 by John Wiley & Sons, Inc.     

Ab initio study of the low-lying electronic states of the CH2NO2 radical

Ab initio electronic structures calculations are reported for the four low-lying electronic states X ²B₁, ²B₂, ²A₂, and ²A₁ of the CH₂NO₂ radical. The geometric parameters for the ground-state X ²B₁ are predicted by MRSDCI calculations with a double zeta plus polarization basis set. The vertical excitations energies for these electronic states are determined using MRSDCI /DZ +P calculations at the ground-state equilibrium geometry and in agreement with the recent experimental data obtained via PES of the CH₂NO anion. The oscillator strenghts and the radiative lifetimes for these electronic states and the spin properties for the ground state are calculated based on the MRSDCI wave functions, predicting results in good agreement with available experimental data. © 1994 John Wiley & Sons, Inc.     

AB initio molecular orbital calculations on the pentadienyl radical

In this publication we present the results of ab initio Gaussian orbital calculations of ESR coupling constants in the pentadienyl radical. The results are compared with experiment and with the results of similar semi-empirical calculations.     

AB initio study of the low-lying excited states of cyclopropene by the equations-of-motion method

The lowest singlet and triplet excited states of cyclopropene have been investigated with the equations-of-motion method. The first spectral band is due to a singlet transition arising from π → 3s,3p excitations. The other two band systems are associated with clusters of transitions predominantly of Rydberg nature or strong admixtures of valence-valence and valence-Rydberg excitations. The first triplet transition is mainly of π → π character.     

Ab initio calculation of the potential surfaces for low-lying valence electronic states of the C2H radical

The results of an extensive CI treatment for the three lowest-lying electronic states 1² A′, 2² A′ and 1² A″ of C₂H are reported. Two-dimensional C-C stretching/bending potential surfaces for these species are calculated. Electronic dipole and transition moments are computed as a function of the bond angle and the C-C bond length. The results serve as a starting point in a theoretical analysis of the rovibronic structure of the long-wavelength spectra of C₂H.     

Electronic structure calculations of low-lying electronic states of O3

Configuration-based multi-reference second order perturbation theory (CB-MRPT2) and multi-reference configuration interaction with single and double excitations (MRCISD) have been used to calculate the bending and dissociation potential energy curves (PECs) of ozone. Based on these PECs, equilibrium structures, vertical and adiabatic transition energies of the ground state and several low-lying excited states, as well as intersections and avoided crossings among the states displayed on the PECs are investigated. The energy separation of the open and ring structures and the dissociation energy of the ground state X(1)A(1) are determined by reference-selected MRCISD. Furthermore, one-dimensional cuts along the dissociation reaction coordinate for the lowest four electronic states of O(3) with (1)A' symmetry and possible pre-dissociations are studied. The Hartley band may be pre-dissociable, and the pre-dissociation limit is found to be 3871 cm(-1), which corresponds to symmetric stretching quanta n(ss) ≈ 6.     

An ab initio study of the low-lying electronic states of S3

Accurate calculations of the low-lying singlet and triplet electronic states of thiozone, S(3), have been carried out using large multireference configuration interaction wave functions. Cuts of the full potential energy surfaces along the stretching and bending coordinates have been presented, together with the vertical excitation spectra. The strong experimentally observed absorption around 395 nm is assigned to the 1 (1)B(2) state, which correlates to ground state products. Absorption at wavelengths shorter than 260 nm is predicted to lead to singlet excited state products, S(2) (a (1)Delta(g))+S((1)D). The spectroscopic properties of the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) electronic states of the S(2) radical have also been accurately characterized in this work. The investigations of the low-lying electronic states were accompanied by accurate ground state coupled cluster calculations of the thermochemistry of both S(2) and S(3) using large correlation consistent basis sets with corrections for core-valence correlation, scalar relativity, and atomic spin-orbit effects. Resulting values for D(0)(S(2)+S) and SigmaD(0) for S(3) are predicted to be 61.3 and 162.7 kcal/mol, respectively, with conservative uncertainties of +/-1 kcal/mol. Analogous calculations predict the C(2v)-D(3h) (open-cyclic) isomerization energy of S(3) to be 4.4+/-0.5 kcal/mol.     

Ab initio study of low-lying electronic states of the FNO2 molecule

Ab initio electronic structure calculations are reported for low-lying electronic states, ¹A₁, ¹A₂, ³A₂, ¹B₁, ³B₁, ¹B₂, and ³B₂ of the FNO₂ molecule. Geometric parameters for the ground state ¹A₁ are predicted by MRSDCI calculations with a double-zeta plus polarization basis set. The vertical excitation energies for these electronic states are determined using MRSDCI/DZ+P calculations at the ground-state equilibrium conformation. The oscillator strengths and radiative lifetimes for some electronic states are calculated based on the MRSDCI wave functions. © 1993 John Wiley & Sons, Inc.     

Ab initio calculation of some low-lying electronic excited states of methane

The energies of some low-lying electronic excited states of methane are calculated by using wave functions built up in terms of plane waves modulated by multicenter Gaussian factors. The wave functions of the various states are evaluated by a two steps iterative process. In the first step, each excited orbital is determined while keeping all other rigid; in the second, rearrangement effects are introduced. Final results are in good agreement with experimental data and allow to enhance an assignement hypothesis for the first electronic transitions.     

Ab initio study of low-lying electronic states of SnCl2+

Complete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), and restricted-spin coupled-cluster singles-doubles with perturbative triples [RCCSD(T)] calculations have been carried out on low-lying doublet and quartet states of SnCl2+, employing basis sets of up to aug-cc-pV5Z quality. Effects of core correlation and off-diagonal spin-orbit interaction on computed vertical ionization energies were investigated. The best theoretical estimate of the adiabatic ionization energy (including zero-point vibrational energy correction) to the X2A1 state of SnCl2+ is 10.093+/-0.010 eV. The first photoelectron band of SnCl2 has also been simulated by employing RCCSD(T)/aug-cc-pV5Z potential energy functions and including Duschinsky rotation and anharmonicity.     

Ab initio calculations on low-lying electronic states of SbO2- and Franck-Condon simulation of its photodetachment spectrum

Geometry optimization and harmonic vibrational frequency calculations have been carried out on the low-lying singlet and triplet electronic states of the antimony dioxide anion (SbO2-) employing a variety of ab initio methods. Both large-core and small-core relativistic effective core potentials were used for Sb in these calculations, together with valence basis sets of up to augmented correlation-consistent polarized-valence quintuple-zeta (aug-cc-pV5Z) quality. The ground electronic state of SbO2- is determined to be the X (1)A1 state, with the a (3)B1 state, calculated to be approximately 48 kcal mole(-1) (2.1 eV) higher in energy. Further calculations were performed on the X (2)A1, A (2)B2, and B (2)A2 states of SbO2 with the aim to simulating the photodetachment spectrum of SbO(2) (-). Potential energy functions (PEFs) of the X (1)A1 state of SbO2- and the X (2)A1, A (2)B2, and B (2)A2 states of SbO2 were computed at the complete-active-space self-consistent-field multireference internally contracted configuration interaction level with basis sets of augmented correlation-consistent polarized valence quadruple-zeta quality. Anharmonic vibrational wave functions obtained from these PEFs were used to compute Franck-Condon factors between the X (1)A1 state of SbO2- and the X (2)A1, A (2)B2, and B (2)A2 states of SbO2, which were then used to simulate the photodetachment spectrum of SbO2-, which is yet to be recorded experimentally.     

AB initio calculations of the electronic structure and doubling parameters of BS

Ab initio calculations have been performed to understand the influence of spin—orbit interaction upon the fine structure of the observed valence doublet states of the BS molecules: spin—orbit splittings, γ-type and Λ-type doublings. Configuration interaction is shown to be an essential feature in order to account for the observed data. Other properties of the electronic states (transition energies) have also been calculated.     

Structure of low-lying electronic states of NdO: quantum chemical calculations

Low-lying states of the NdO molecule have been predicted from quantum mechanical complete active-space self-consistent field/multireference configuration interaction/spin-orbit calculations. 54 states labeled through the quantum number Omega(+/-) have been determined in the excitation energy range of approximately 1 eV. For each state molecular constants T(e), T(v), omega(e), deltaG(v), R(e), B(e), and B(v) have been calculated. All these states display nearly identical principal structural characteristics: equilibrium internuclear distance and vibrational frequency. Calculated values of T(v), deltaG(v), and B(v) agree satisfactorily with experimental values available for nine electronic states among the 54 considered. The feasibility of a statistical representation of the low-lying states of NdO is considered.     

Ab initio calculations of surface electronic states in indium oxide

A slab approach in the framework of ab initio calculations was applied to study surface electronic states in In₂O₃ crystal. Density functional theory (DFT) calculations were carried out employing the WIEN 2k code and using the full potential method with Augmented Plane Waves + local orbitals (APW+lo) formalism. Total and partial DOS (Density of States) were calculated for In and O atoms in two upper (110) surface layers. Comparison of total and partial DOS allowed determining a contribution of electronic states of different In and O surface atoms into formation of surface electronic spectra and corresponding chemical bonds. A dominant ionic character of chemical bonds in In₂O₃ is found. Calculations were performed for three slab models with different geometry parameters. It was shown that an optimal ratio between the whole vertical size of a supercell and the vertical size of atomic cluster has to be chosen. The size of vacuum region in the slab model influences significantly on the reliability of calculated characteristics of the surface electronic structure. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.     

Ab initio calculations of the lowest electronic states in the CuNO system

The lowest singlet and triplet electronic levels of the A' and A" symmetry species of the neutral copper-nitrosyl (CuNO) system are calculated by ab initio methods at the multi-reference configuration interaction (MRCI) level of theory with single and double excitations, and at the coupled cluster level of theory with both perturbational (CCSD(T)) and full inclusion of triple excitations (CCSDT). Experimental data are difficult to obtain, hence the importance of carrying out calculations as accurate as possible to address the structure and dynamics of this system. This paper aims at validating a theoretical protocol to develop global potential energy surfaces for transition metal nitrosyl complexes. For the MRCI calculations, the comparison of level energies at linear structures and their values from C(2v) and C(s) symmetry restricted calculations has allowed to obtain clear settings regarding atomic basis sizes, active orbital spaces and roots obtained at the multi-configurational self-consistent field (MCSCF) level of theory. It is shown that a complete active space involving 18 valence electrons, 11 molecular orbitals and the prior determination of 12 roots in the MCSCF calculation is needed for overall qualitatively correct results from the MRCI calculations. Atomic basis sets of the valence triple-zeta type are sufficient. The present calculations yield a bound singlet A' ground state for CuNO. The CCSD(T) calculations give a quantitatively more reliable account of electronic correlation close to equilibrium, while the MRCI energies allow to ensure the qualitative assessment needed for global potential energy surfaces. Relativistic coupled cluster calculations using the Douglas-Kroll-Hess Hamiltonian yield a dissociation energy of CuNO into Cu and NO to be (59 ± 5) kJ mol(-1) ((4940 ± 400) hc?cm(-1)). Favorable comparison is made with some of previous theoretical results and a few known experimental data.     

AB initio calculations and matrix ftir studies of the sulfur trioxide radical anion

The structures and vibrational frequencies of SO⁻₃ (C_3v) and SO⁻₂ (C_2v) have been calculated at the UHF SCF/3-21 + G¹ level. By cocondensation of Cs atoms and SO₃ in an Ar matrix the FTIR spectrum of Cs₄SO₃ has been measured. The molecule is proposed to have C_s symmetry with SO₃ binding to Cs in a bidentate fashion.     

Vibronic dynamics in the low-lying coupled electronic states of methyl cyanide radical cation

Static and dynamic aspects of the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) interactions between the ground and first excited electronic states of the methyl cyanide radical cation are theoretically investigated here. The latter involves construction of a theoretical model by ab initio computation of electronic potential energy surfaces and their coupling surfaces and simulation of the nuclear dynamics employing time-independent and time-dependent quantum mechanical methods. The present system represents yet another example belonging to the (E + A)  e JT–PJT family, with common JT and PJT active degenerate (e) vibrational modes. The theoretical results are found to be in very good accord with the recent experimental data revealing that the JT interactions are particularly weak in the ground electronic manifold of methyl cyanide radical cation, On the other hand, the PJT interactions of this ground electronic manifold with the first excited electronic state of the radical cation are stronger which cause an increase of the spectral line density. The effect of deuteration on the JT–PJT dynamics of the methyl cyanide radical cation is also discussed.     

The electronic structure of carbon suboxide from ESCA and AB initio calculations

The ESCA spectrum of C₃O₂ excited by Mg K_α radiation, 1253.6 eV, has been obtained for both gaseous and solid samples. The chemical shifts of the C1s and O1s levels have been used to calculate the gross atomic charges. The valence region of the spectrum has been recorded and the ordering of the orbitals has been decided on the basis of an ab initio calculation and the intensities of the observed peaks. An unusually intense shake-up spectrum has also been observed and is discussed. The relative spacing of the valence peaks has been found to be different for the solid and gas.