Parametrization of a Heitler–London valence bond Hamiltonian from complete-active-space self-consistent-field computations: An application to chemical reactivity

A simple practical procedure is proposed for the transformation of a complete-active-space self-consistent-field (CAS-SCF) wave function to Heitler–London valence bond space via the construction of an effective (Heisenberg) Hamiltonian. With such a procedure, the computed Heitler–London parameters Q and Kij can be used for a posteriori rationalization of the bonding effects in molecular structures. In particular, the diabatization of the adiabatic potential surface, first proposed by Evans can now be accomplished in a rigorous way. The technique is applied to some previously computed transition structures for cycloaddition reactions.