Unified theoretical treatment of analytic first and second energy derivatives in open-shell Hartree—Fock theory

Abstract

Second derivatives of polyatomic potential energy hypersurfaces are of widespread importance to problems in theoretical chemistry. A formalism is presented which allows the analytic evaluation of energy second derivatives from restricted Hartree—Fock wavefunctions for arbitrary closed- or open-shell molecular systems. The present method makes use of previously reported integral second-derivative techniques and earlier advances in the solution of the open-shell coupled perturbed Hartree—Fock equations. The applicability of the new method is demonstrated in studies of the first excited singlet state of formaldehyde, for which the equilibrium geometry and vibrational frequencies have been determined with two different basis sets.