Interaction-optimized virtual orbitals, I. External double excitations

Virtual orbitals for correlated electronic wavefunctions may be determined by maximizing the interaction between a reference function and each virtual configuration. The interaction-optimized virtual orbitals for external double excitations are determined by the eigenfunctions of exchange-like operators. The operator and matrix forms of these equations are presented for intraorbital pairs, and for interorbital pairs with direct product spin couplings. The resulting virtual orbitals provide a natural representation to first order in the wavefunction.