A hybrid MC-SCF method: generalised valence bond (GVB) with complete active space SCF (CASSCF)

A hybrid GVB and CASSCF method is proposed, GVBCAS, in which a full CAS CI is performed amongst the active orbitals (the CAS part), and GVB type excitations are allowed amongst a set of GVB orbitals. We demonstrate that the computational effort required for the energy and gradients is little more than that for a CASSCF calculation with the strongly occupied GVB orbitals allocated to the inactive space, and substantially less than that for a full CASSCF in the space of active + GVB orbitals. The efficacy of the method is illustrated with an example.