Combining perturbation theory techniques with variational CI calculations to study molecular excited states

Abstract

The accurate treatment of molecular excited states is based on a matrix solution of the electronic Schrdinger equation in a suitable AO basis which relies heavily on the existence of a good approximate description of a given eigenfunction prior to the solution of a secular equation. The manner in which one or more roots corresponding to excited states can be traced from a rather simple starting point to the final results obtained from a large-scale CI treatment is outlined. In such calculations it is very useful to have a measure of the energy-lowering capability of configurations which differ by at most a double excitation from a series of reference configurations which represent the dominant contributions to one or more electronic states. The efficient computation of the required Hamiltonian matrix elements for this purpose, as well as for the construction of the final secular equations, can be achieved through the use of the table CI method, which does not rely on any special relationship between configurations to complete its work. Employing CI expansions of length no greater than 10000-2oooO terms is very advantageous for the calculation of special properties of the resulting wavefunctions, especially those of spin-dependent quantummechanical operators. W i t h the help of perturbative corrections it is possible to obtain energy results with such methods which are accurate to within 0.01 hartree of the full CI limit [R. A. Phillips, R. J. Buenker, P. J. Bruna, and S. D. Peyerimhoff, Chem. Phys. 84, 11 (1984)], whose error is small compared to that inherent in the use of conventional double-zeta-plus-polarization AO basis sets, therefore making further reduction of the CI errors almost inconsequential. Numerous examples for the calculation of the energy and properties of molecular excited states are surveyed.

The electronic spectrum of the SiC radical: A theoretical study

“…Some spectroscopic information of X 3 P, A 3 R À , b 1 P, and c 1 D states of SiC and the 2 P state of SiC À have been reported. The millimeter-wave rotational spectra with hyperfine structure of two stable isotopes with nuclear spin, namely 29 SiC and Si 13 C, were detected in the ground state [20,21]. The vibrational and adiabatic ionization energies and electron affinities of Si n C and Si n O (n = 1-3) molecules have been reported by Boldyrev et al [22] from large-scale ab initio calculations at different levels of correlation.…”

“…The symmetry adapted SCF-MOs are subsequently used for the generation of configurations in the CI calculations. Throughout the calculations we have employed the MRDCI methodology of Buenker and coworkers [27][28][29][30][31][32][33] which uses perturbative correction and energy extrapolation techniques. The table-CI algorithm is used to handle open shell configurations which appear due to the excitation process.…”

The hyperfine coupling constants of 19F: An ab initio MRD‐CI basis set study

“…The UHF program and spin annihilation techniques were described in paper I. For the CI calculations, MRD-CI [14][15][16][17][18] wave functions were used, together with the hyperfine programming package as described by Engels et al [7]. There, the integrals for the hyperfine operators are obtained according to Chandra and Buenker [ 191. Molecular orbitals obtained for the ground state of Fi have been used for the CI expansion.…”

The formation of SiH+, PH+, and SH+ by radiative association

“…The potential curves and electric dipole transition moments are calculated by the multireference singleand double-excitation (MRD-CI) configuration interaction method (Buenker & Peyerimhoff 1974, 1975Buenker 1986), with configuration selection and energy extrapolation using the Table CI algorithm (Buenker 1980(Buenker , 1981Buenker & Phillips 1985). During the CI excitation procedure, the five lowest molecular orbitals corresponding to 1s, 2s, and 2p orbitals of the heavy atoms are always doubly occupied, whereas some high virtual orbitals are discarded.…”