A modified variation-perturbation approach to zero-point vibrational motion

We present a detailed investigation of the perturbation approach for calcul ating zero-point vibrational contributions to molecular properties. It is d emonstrated that if the sum of the potential energy and the zero-point vibr ational energy is regarded as an effective potential energy, the leading co ntribution to the first-order wave function vanishes in the perturbation ap proach. Two different perturbation approaches have been investigated numeri cally by calculations of some magnetic properties for a few diatomic molecu les and the results obtained have been compared to the exact numerical resu lts. It is shown that only a few terms need to be included in a perturbatio n expansion to obtain an accuracy in the calculated rovibrational contribut ion better than the quality normally obtained for the potential and propert y surfaces in electronic structure calculations.