A multireference perturbation method using non-orthogonal Hartree-Fock determinants for ground and excited states

In this article we propose the Delta SCF(2) framework, a multireference strategy based on second-order perturbation theory, for ground and excited electronic states. Unlike the complete active space family of methods, Delta SCF(2) employs a set of self-consistent Hartree-Fock determinants, also known as Delta SCF states. Each Delta SCF electronic state is modified by a first-order correction from Moller-Plesset perturbation theory and used to construct a Hamiltonian in a configuration interactions like framework. We present formulas for the resulting matrix elements between nonorthogonal states that scale as (NoccNvirt3)-N-2. Unlike most active space methods, Delta SCF(2) treats the ground and excited state determinants even-handedly. We apply Delta SCF(2) to the H-2, hydrogen fluoride, and H-4 systems and show that the method provides accurate descriptions of ground- and excited-state potential energy surfaces with no single active space containing more than 10 Delta SCF states. (C) 2013 AIP Publishing LLC.