Iterative active-space selection for vibrational configuration interaction calculations using a reduced-coupling VSCF basis

We adapt a variation–perturbation method to perform vibrational configuration interaction (VCI) calculations on large molecular systems. Starting from a self-consistent vibrational wave function, we use the vibrational configuration interaction with perturbation selected interactions (VCIPSI) algorithm to select an active VCI space iteratively. We then extend this approach to use a reduced-coupling description of potential energy surfaces. The accuracy of the method is tested on methane and benzoic acid molecules. The significant reduction in the size of the VCI basis obtained using the VCIPSI scheme does not affect the quality of the computed vibrational frequencies and reduces computational time dramatically.