A new MO-VB approach for the determination of intermolecular forces. Theory and application to the He2, He-CH4 and He-H2O systems

We present the improvement of a previously developed strategy for the evaluation of intermolecular forces. The approach defines a variational VB (valence bond) wave function, consisting of single and double excitations from the SCF-MI (self-consistent field for molecular interactions) determinant. The central idea of the method is the determination of optimal virtual orbitals, to contract the virtual space spanned by all singly and doubly excited localized configurations, by means of an iterative optimization procedure. The performance of the strategy is tested by comparison with results where the full virtual space is considered, and the entire approach is also compared with more conventional quantum chemical methods. Test calculations on three weakly interacting complexes, namely, He2, He-CH4, and He-H2O, are presented. Whatever the system studied, we found an overall agreement between VB, MP4, and CCSD(T) results. The VB well depths estimates are somewhat larger than MP4 and CCSD(T) ones.