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

In this article we propose the ΔSCF(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, ΔSCF(2) employs a set of self-consistent Hartree-Fock determinants, also known as ΔSCF states. Each ΔSCF electronic state is modified by a first-order correction from Mo̸ller-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 N(occ)(2)N(virt)(3). Unlike most active space methods, ΔSCF(2) treats the ground and excited state determinants even-handedly. We apply ΔSCF(2) to the H2, hydrogen fluoride, and H4 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 ΔSCF states.