Ab initio non-orthogonal approaches to the computation of weak interactions and of localised molecular orbitals for QM/MM procedures

The research carried out by our group in the framework of the VB theory is reviewed. The modification of the Roothaan equations to avoid the basis set superposition error (BSSE) at the Hartree-Fock level of theory is presented. Significant applications of the resulting scheme, called Self Consistent Field for Molecular Interactions (SCF-MI), are summarised. The successive development of the SCF-MI method to properly take account of electron correlation effects by a compact multistructure size consistent Valence Bond (VB) wavefunction is described. The central idea of the method is that of expanding the orbitals in the partitioned basis sets located on each fragment so as to exclude BSSE in an a priori fashion, while taking properly into account orbital and geometry relaxation effects and the natural non orthogonality of the MO's of the interacting fragments, The very recent extension of the SCF-MI formulae to the determination of extremely localised molecular orbitals (ELMO) is also reviewed.