Hamiltonian and overlap matrix elements between half-projected Slater determinants

Abstract

We obtain expressions for the Hamiltonian and overlap matrix elements between half-projected Slater determinants of the type “different orbitals for different spins” and differing by one or two orbitals. We consider the use of these expressions in the construction of direct orbital optimization schemes in the half-projected Hartree-Fock method (HPHF), in the study of the instability of the Hartree-Fock solution, in the study of the singly-excited configuration interaction and random phase methods and in the interpretation of the static response function in the HPHF method.