Perturbation-theory method of calculating the energies and excitation energies of atomic, molecular, and solid-state systems

We investigate a general, approximate method for solving the many-electron Schrödinger equation for a molecule or a molecular fragment representing a solid. The method chosen is the unrestricted Hartree-Fock method augmented by a size-consistent many-body perturbation-theory correction. For simplicity, a single-reference development is presented. The extension to multireference formalisms is straightforward if tedious. A few sample results are obtained to illustrate the potential of the method. © 1986 The American Physical Society