Semiempirical NDDO/MC calculations of the excited states of the chromate ion

A new semiempirical method of calculating the excited states of transition metal complexes is developed. This technique uses the configuration interaction and semiempirical NDDO/MC methods to obtain the ground state of a set of Slater type valence spd-orbitals chosen from the optical spectra of transition metals together with the corresponding core integrals. The method is tested in calculations of the electronically excited states of the chromate ion. Good agreement with the experimental energies of vertical transitions and the results of ab initio calculations is achieved.     

Theoretical investigation of Wulf and Chappuis bands in the spectrum of ozone

The ground state and some lowest excited states of ozone are calculated by the semiempirical MNDO method using configuration interaction to explain the Wulf absorption band and photodissociation of ozone. The results of calculations show that³A₂(1³A′’) is the lowest excited triplet state of O₃; a transition to this state from the ground X¹ A₁ state is responsible for the weak Wulf absorption. The oscillator strength (f = 3.2·10^-7) and the radiative lifetime of the A₂ state (Τ = 0.01 s) are in agreement with recent ab initio calculations. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 6, pp. 1067–1073, November–December, 1997.     

A theoretical study of photoinduced electron transfer between tetracyanoethylene and arene

 Ab initio calculations have been performed to investigate the state transition in photoinduced electron transfer reactions between tetracyanoethylene and biphenyl as well as naphthalene. Face-to-face conformations of electron donor–acceptor (EDA) complexes were selected for this purpose. The geometries of the EDA complexes were determined by using the isolated optimized geometries of the donor and the acceptor to search for the maximum stabilization energy along the center-to-center distance. The correction of interaction energies for basis set superposition error was considered by using counterpoise methods. The ground and excited states of the EDA complexes were optimized with complete-active-space self-consistent-field calculations. The theoretical study of the ground state and excited states of the EDA complex in this work reveals that the S₁ and S₂ states of the EDA complexes are charge–transfer (CT) excited states, and CT absorption which corresponds to the S₀→S₁ and S₀→S₂ transitions arise from π−π^* excitation. On the basis of an Onsager model, CT absorption in dichloromethane was investigated by considering the solvent reorganization energy. Detailed discussions on the excited state and on the CT absorptions were made. Received: 30 April 2001 / Accepted: 18 October 2001 / Published online: 9 January 2002     

An analysis of predictions by the semi-empirical MNDO molecular orbital method for some aspects of molecular energetics

Calculations using the MNDO HE molecular-orbital method predict that NH₄, PH₄ and H₃S are stable free radicals, and that local minima exist for H₃O and H₂Cl. The tendency of MNDO to overestimate the stability of such systems is traced to its neglect of overlap integrals. Results for H⁺₂ (both ground and excited states) support this conclusion. Stable radicals with formulas BeH₃ and BH₄ are also predicted. Analysis of hydrocarbon thermochemistry indicates that MNDO fails to predict the correct sign for the relative magnitude of bond interactions, whereas ab initio calculations give the correct sign and approximately the correct magnitude. Finally, it is shown that the apparent superiority of MNDO over ab initio MO calculations in predicting molecular energetics is illusory when the methods are compared on the same basis.     

Accurate Ab Initio Calculations for LiH and its Ions, LiH+ and LiH−

Ab initio calculations were performed for LiH using a pseudopotential approach with CPP corrections and huge basis sets on both atoms. A wide range of ^1,3Σ⁺ electronic adiabatic states have been investigated, from the ground state up to those dissociating into Li(5p)+H. Permanent and transition electric dipole moments are also considered for the first few excited states. Comparison with experiments and recent all-electron calculations, reveals an excellent global accuracy, only the bottom of the ground state being better described by all-electron approaches. Using almost identical basis sets, coupled cluster all-electron calculations are performed for the ground states of LiH⁺, LiH⁻ and LiH. High care has been given to the correct relative position of the asymptotes, allowing for this rather complete set of accurate ab initio data to be useful for further molecular physics studies.     

Quasirelativistic valence ab initio calculation of the potential-energy curves for Cd–rare gas atom pairs

The results of large-scale valence ab initio calculations of the potential-energy curves for the ground states and several excited states of Cd–rare gas (RG) van der Waals molecules are reported. In the calculations, Cd²⁰⁺ and RG⁸⁺ cores are simulated by energy-consistent pseudopotentials, which also account for scalar-relativistic effects and spin-orbit interaction within the valence shell. The potential energies of the Cd–RG species in the ΛS coupling scheme have been evaluated by means of ab initio complete-active-space multiconfiguration self-consistent-field (CASSCF)/CAS multireference second-order perturbation theory (CASPT2) calculations with a total 28 valence electrons, but the spin-orbit matrix has been computed in a reduced configuration interaction space restricted to the CASSCF level. Finally, the Ω potential curves are obtained by diagonalization of the modified spin-orbit matrix (its diagonal elements before diagonalization substituted by the corresponding CASPT2 eigenenergies). The calculated potential curves, especially the spectroscopic parameters derived for the ground states and several excited states of the Cd–RG species are presented and discussed in the context of available experimental data. The theoretical results exhibit very good agreement with experiment. Received: 20 April 2000 / Accepted: 1 September 2000 / Published online: 21 December 2000     

Potential energy curves of several excited states of the Ne2* excimer: Assignment of the transient absorption spectra of the excimer

Most of the excited states of Ne₂, which are correlated with the Rydberg state transitions 2p → 3s, 3p, and 4s of Ne, are studied by ab initio CI calculations. Two transient absorption spectra from the lowest excimer state Σ_u⁺ recently observed by Arai et al., are discussed on the basis of calculated potential energy curves. Possible assignments are presented. The calculated transition energies are in good agreement with the observed ones.     

Rydberg character of the higher excited states of free-base porphin

 The Rydberg character of the excited states of free-base porphin (FBP) has been investigated by the ab initio configuration interaction singles (CIS) method and the state-averaged complete-active-space self-consistent-field method. Double-zeta basis sets augmented with s, p, and d Rydberg functions and d polarization functions have been employed. Two types of molecular orbitals sets, the restricted Hartree–Fock molecular orbitals obtained for the ground state (¹A _g ) and for the cation state (²A _u ), have been used in the CIS calculations. All the calculations show that Rydberg-type excitations play important roles especially in the N bands. In this article we propose applying the model of a perturbed Rydberg series to interpret the excited states of FBP. By using this model, we have succeeded in analyzing the characteristics of the excited states as well as the experimental oscillator strengths, which have considerable magnitude even in the higher excited states. Received: 27 November 2000 / Accepted: 11 April 2001 / Published online: 27 June 2001     

A density functional study of the electronic spectrum of permanganate

Multiplet splittings for six excited electronic configurations of the permanganate ion, MnO₄⁻, are calculated. Earlier density functional calculations on the same subject are improved upon by the numerical evaluation of some two-electron integrals to resolve certain multideterminantal states. Excellent agreement with the experimental spectrum is obtained, and a reassignment of bands in the 25,000–35,000 cm⁻¹ range is proposed. Fully symmetric (a₁) vibrational frequencies are calculated, and the origin and magnitude of the most significant Jahn-Teller distortions of the excited states are discussed. © 1996 John Wiley & Sons, Inc.     

Calculation of ground- and excited-state potential energy curves for barium-rare gas complexes in a pseudopotential approach

Adiabatic potential curves for the ground state and several low-lying excited states of the barium atom interacting with Ne, Ar, Kr and Xe have been obtained from valence ab initio configuration-interaction calculations. Atomic cores are replaced by scalar-relativistic l-dependent pseudopotentials, while core-polarization potentials are used for describing correlation contributions of the rare-gas atoms and the Ba²⁺ cores. Implications of the resulting potential curves for the interpretation of experimental data are discussed, together with first applications of the curves for calculating absorption profiles of the (6s ²)¹S→(6p)¹P Ba transition. Received: 7 April 1998 / Accepted: 27 July 1998 / Published online: 12 October 1998     

Evaluation of Hylleraas-CI atomic integrals by integration over the coordinates of one electron. I. Three-electron integrals

A method to evaluate the nonrelativistic electron-repulsion, nuclear attraction and kinetic energy three-electron integrals over Slater orbitals appearing in Hylleraas-CI (Hy-CI) electron structure calculations on atoms is shown. It consists on the direct integration over the interelectronic coordinate r _ij and the sucessive integration over the coordinates of one of the electrons. All the integrals are expressed as linear combinations of basic two-electron integrals. These last are solved in terms of auxiliary two-electron integrals which are easy to compute and have high accuracy. The use of auxiliary three-electron ones is avoided, with great saving of storage memory. Therefore this method can be used for Hy-CI calculations on atoms with number of electrons N ≥ 5. It has been possible to calculate the kinetic energy also in terms of basic two-electron integrals by using the Hamiltonian in Hylleraas coordinates, for this purpose some mathematical aspects like derivatives of the spherical harmonics with respect to the polar angles and recursion relations are treated and some new relations are given.     

Determination of the dipole moment of thioformaldehyde in the a3A2 excited state by laser phosphorescence excitation spectroscopy

Doppler-limited phosphorescence excitation spectra have been recorded at various electric fields for two rotational transitions in the $\text{a}$³A₂-$\text{X}$¹A₁ 0-0 band of H₂CS. Stark splittings were resolved, and were used to determine the dipole moment in the excited electronic state. The value found, 0.57(3) D, is of the order expected by comparison with dipole moments determined for other states of H₂CS, but rather lower than that predicted by ab initio calculations.     

Nonadiabatic coupling and diabatic electronic population dynamics on 11A2 and 11B1 states of ozone molecule

In this work, we examine nonadiabatic population dynamics for 1¹B₁ and 1¹A₂ states of ozone molecule (O₃). In O₃, two lowest singlet excited states, ¹A₂ and ¹B₁, can be coupled. Thus, population transfer between them occurs through the seam involving these two states. At any point of the seam (conical intersection), the Born-Oppenheimer approximation breaks down, and it is necessary to investigate nonadiabatic dynamics. We consider a linear vibronic coupling Hamiltonian model and evaluate vibronic coupling constant, diabatic frequencies for three modes of O₃, bilinear and quadratic coupling constants for diabatic potentials, displacements, and Huang-Rhys coupling constants using ab initio calculations. The electronic structure calculations have been performed at the multireference configuration interaction and complete active space with second-order perturbation theory with a full-valence complete active space self-consistent field methods and augmented Dunning's standard correlation-consistent-polarized quadruple zeta basis set to determine ab initio potential energy surfaces for the ground state and first two excited states of O₃, respectively. We have chosen active space comprising 18 electrons distributed over 12 active orbitals. Our calculations predict the linear vibronic coupling constant 0.123 eV. We have obtained the population on the 1¹B₁ and 1¹A₂ excited electronic states for the first 500 fs after photoexcitation.     

Evaluation of Orbital- and Ground State Energies of Some Open- and Closed-Shell Atoms over Integer and Noninteger Slater Type Orbitals

Ab initio calculations of the orbital and the ground state energies of some open- and closed-shell atoms over Slater type orbitals with quantum numbers integer and Slater type orbitals with quantum numbers noninteger have been performed. In order to increase the efficiency of these calculations the atomic two-electron integrals were expressed in terms of incomplete beta function. Results were observed to be in good agreement with the literature.     

Ab initio energies and tunneling lifetimes of the doubly charged AH2+ (A = Mg—Ar) diatomics

Potential energy curves for the ground and low-lying excited states of the AH²⁺ (A = Mg—Ar) dications have been calculated using high-level ab initio methods with large atomic orbital basis sets. Quasi-bound potential energy curves with local minima and deprotonation barriers have been found for most of the dications studied. The energies, tunneling lifetimes, and widths of the quasi-bound states have been calculated by numerical solution of the radial Schrodinger equation using the Numeov method. All these dications except ArH²⁺ have low-lying states which support quasi-bound vibrational states. The ArH²⁺ dication has a ²∏_i potential energy curve with a minimum so shallow that it does not support any quasi-bound vibrational states. Results of our calculations are compared with previous ab initio calculations and available experimental data. © 1995 John Wiley & Sons, Inc.     

Ab initio configuration interaction calculations on the hydrogen molecular excimer,H*4

Results of ab initio configuration interaction calculations are presented for a simple model of molecular excimers, (H₄)^*. Preliminary SCF calculations as well as several levels of CI are discussed for the lowest three singlet and triplet states of H₄. Square, rectangular, and regular trapezoid geometries of H₄ are examined. The results of this study are significantly better in a quantitative sense than earlier work on the excited states of H₄.     

An ab initio SCF and CI study of ketene imine

The electronic structures of the ground and excited states of ketene imine (_H^HCCNH) have been studied by ab initio SCF and CI calculations. The nucleophilic nature of the β carbon with respect to nitrogen has been discussed using calculated electrostatic potentials and by calculated energy differences between the parent and protonated species. The electronically excited ¹A″ and ³A″ states are found to be almost degenerate.     

Analysis of electronic states and energy level structure of uranyl in compounds

An effective operator Hamiltonian has been developed for evaluating the excited states of uranyl ion in compounds. Including free-ion and crystal-field interactions, the parametrized Hamiltonian is able to reproduce the energy level structure in good agreement with experimental measurements on uranyl in chloride compounds and nitride complexes. It is shown that the low-lying excited states belong to a nonbonding σf configuration (therefore, one Slater integral of G(3) is sufficient to account for the Coulomb electron exchange interaction) and that the spin-orbit coupling surpasses the electrostatic interactions in energy level splitting. In comparison with previously reported ab initio calculations, the present studies find that the Coulomb interactions in uranyl were excessively evaluated in the first-principle calculations.     

Visible-laser-induced chemiluminescence of NaHg red excimer bands

Visible argon ion laser lines were used to induce the chemiluminescence of the NaHg excimer. Sodium dimers, excited into theB ¹Π _u state, produced in reactive collision with Hg atoms the NaHg excimers in the first excited electronic states. A qualitative analysis of the origin of the observed spectra, based on ab initio NaHg potential curves calculations, is presented.     

Highly accurate relativistic universal Gaussian basis set for Dirac–Fock–Breit calculations

Ion mobilities of H₂O⁺ drifting in helium are calculated and compared with experiment. These calculations employ global potential energy surfaces of the H₂O⁺–He complex, which in the present case were calculated ab initio at the unrestricted MP2 level of theory using a basis set of aug‐cc‐pVTZ quality, and treating the ion as a rigid body. Details are presented of the general characteristics of both the ground and first‐excited electronic states of the complex. Although only the ground‐state surface was used for the mobility calculations, the ab initio determination of the ground state necessitated the inclusion of the first‐excited state owing to the presence of a crossing between the two. This crossing is also described. Mobilities calculated from the global surfaces are in good agreement with experiment. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005