Generalized valence bond solutions from a constrained coupled cluster method Original Research Article

The GVB-PP wavefunction is cast into a coupled cluster form with the coupled cluster operator constrained to intrabond double excitations. Following the coupled cluster ansatz, where the trial wave function is assumed to satisfy the Schrödinger equation, projections onto the reference and doubly excited configurations are used to determine the energy and coefficients respectively. A decoupling of these equations results, allowing analytical solutions. The active orbital space is simultaneously optimized to produce the lowest energy. This is carried out efficiently using a procedure previously developed by Head-Gordon and Pople for the direct optimization of a Hartree-Fock wave function. Preliminary calculations for singlet/triplet states and bond dissociations show that although this method is strictly nonvariational, local minima are found which lie within a few tenths of a millihartree of the true GVB-PP energy.