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 presente d. Significant applications of the resulting scheme, called Self Consistent Field for Molecular Interactions (SCF-MI), are summarised. The successive development of the SCF-Ml method to properly take account of electron corre lation effects by a compact multistructure size consistent Valence Bond (VB ) wavefunction is described. The central idea of the method is that of expa nding the orbitals in the partitioned basis sets located on each fragment s o as to exclude BSSE in an a priori fashion, while taking properly into acc ount orbital and geometry relaxation effects and the natural non orthogonal ity of the MO's of the interacting fragments. The very recent extension of the SCF-MI formulae to the determination of extremely localised molecular o rbitals (ELMO) is also reviewed. (C) 2001 Elsevier Science B.V. All rights reserved.