Generalization of analytic configuration interaction (CI) gradient techniques for potential energy hypersurfaces, including a solution to the coupled perturbed Hartree–Fock equations for multiconfiguration SCF molecular wave functions

Abstract

Analytic configuration interaction gradient techniques for multiconfiguration self-consistent field (MCSCF) wave functions have been developed by solving the coupled perturbed equations for the generalized Hartree–Fock operator. The method has been implemented in conjunction with the loop-driven graphical unitary group approach for the determination of correlated wave functions. This theoretical approach is in principle applicable to any type of configuration interaction (CI) wave function constructed from orbitals determined from a rigorous SCF or MCSCF procedure. Initial applications have been to CI wave functions involving all single and double excitations relative to an open-shell singlet restricted Hartree–Fock reference function. The specific example chosen was the first excited singlet state (b 1B1) of CH2, for which the equilibrium geometry and harmonic vibrational frequencies have been predicted with a double zeta plus polarization basis set.