Self-Consistent Field Theory of Weakly Bonded Systems

Roothaan equations have been modified with the aim of avoiding basis set superposition error (BSSE) at the Hartree-Fock level of theory. The resulting scheme, called SCF-MI (Self Consistent Field for Molecular Interactions), underlines its special suitability for the computation of intermolecular interactions. In the present work we report with some details the generalisation of the theory to the case of K interacting fragments. The method provides a complete a priori elimination of the BSSE while taking into account the natural non orthogonality of the MO's of the interacting fragments. Several application examples of the SCF-MI approach are presented and discussed. We mainly concentrate on large van der Waals systems (nucleic acid base pair complexes) and water molecular cluster studies.