A multi-reference approach to energy decomposition for molecular interactions

Abstract

A method is proposed for the analysis of the components of molecular interaction energy using a multi-reference expansion of the wavefunction. The method can be applied to structural and reactivity problems. The MC-SCF orbitals of the isolated fragments are used as the basis for the construction of the wavefunctions of the molecule as a linear combination of fragment configurations. The energy decomposition is based upon the definition of no-bond and charge transfer configurations and is defined relative to a suitable reference state. The energy of an anti-symmetrized product of isolated fragment wavefunctions gives the electrostatic and exchange repulsion energy, while the subsequent inclusion of the charge transfer configurations gives the valence charge transfer associated with bond formation. Orbital mixing between core and valence, core and virtual, and valence and virtual are then computed using second order perturbation theory or MC-SCF. For illustrative purposes the method has been applied to the analysis of the rotational barrier in ethane, the dimerization of methylene and the reaction of methylene and silylene.