The secular equation/perturbation theory method for calculating spectra of van der Waals complexes

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Abstract
A new method for calculating vibrational wave functions and level energies for van der Waals molecules is proposed. It consists of a minimal secular equation calculation, including only strongly coupled basis functions, followed by perturbation theory corrections to take account of the effects of weakly coupled channels. The method is very efficient, and gives results accurate to ±0.001 cm−1 for Ar–H2. It is also readily applicable to predissociating states of van der Waals molecules, and gives calculated level widths more accurate than any previous approximate method. The method obviates the resonance search problem associated with direct coupled channel methods for characterizing predissociating states, and is readily extendable to more strongly anisotropic systems.