Quasidegenerate many-body perturbation theory of CH2

The effective valence shell Hamiltonian (Hν) formulation of quasidegenerate many-body perturbation theory is applied to the CH2 molecule and its positive ion to investigate the ability of the method to describe large spaces having a wide range of orbital energies and patterns of quasidegeneracy that vary greatly with molecular geometry. Sensitive tests are provided by the adiabatic singlet–triplet energy difference, by the lowest singlet excitation energy, by the ionization potentials to a series of ion states, and by the question of whether accurate energies emerge simultaneously for a whole set of valence and ion states from a single Hν computation. Computations assess the dependence of Hν calculations on the choice of orbitals and orbital energies, the only degrees of freedom available to the Hν method once the valence space has been prescribed. Third order corrections are analyzed in preparation for the computations of the following paper that utilize the present calculations as a guide in studying the theoretical basis of the pairwise additivity assumption of semiempirical all valence electron methods.