Analytic configuration interaction (CI) gradient techniques for potential energy hypersurfaces. A method for open-shell molecular wave functions

A formalism is presented for the determination of analytic energy first derivatives for the most common types of open-shell correlated wave functions. These are the cases for which the electronic structure is qualitatively understood in terms of a set of molecular orbitals which are eigenfunctions of Roothaan’s restricted Hartree–Fock (RHF) operator. Using these RHF orbitals, configuration interaction (CI) wave functions of broad generality are included in the formalism. The method has been implemented in conjunction with the loop-driven graphical unitary group approach. Application to the vibrational frequencies of methylene suggests that the triplet state has 0.5 kcal more zero-point vibrational energy than does the lowest singlet state.