The augmented secular equation method for calculating spectra of van der Waals complexes. Application to the infrared spectrum of Ar–HCl

Abstract

The recently proposed secular equation–perturbation theory (SEPT) procedure for calculating spectra of van der Waals molecules is tested on the strongly anisotropic system Ar–HCl. It is found to be more accurate than other available approximate schemes, but the convergence with respect to basis set size is slow. An alternative method, which uses the perturbation correction from a SEPT calculation to augment the original secular equation basis set, is then proposed and tested, and is found to converge considerably faster. This method is then used to calculated spectroscopic properties for various excited states of Ar–HCl, and the results are compared with the recent experimental data of Marshall et al.(J. Chem. Phys., 1985, 83, 4924) and Ray et al.(J. Chem. Phys., 1986, 84, 1171).