Full configuration interaction benchmarks for the (X)over-tilde(3)B(1), (a)over-tilde(1)A(1), (b)over-tilde(1)B(1) and (c)over-tilde(1)A(1) states of methylene

Previous experimental and theoretical work on the four lowest lying electronic states of methylene is summarized and new benchmark energies, geometries, dipole moments and harmonic vibrational frequencies are obtained by solving the electronic Schrodinger equation exactly, given a double-zeta plus polarization (DZP) basis jet and restricting the core orbital to remain doubly occupied. These full configuration interaction (CI) results are used to evaluate: several approximate treatments of electron correlation. The predictions of a CI procedure including all single, double, triple and quadruple excitations (CISDTQ) are virtually identical to the full CI results for all but the EIA I state, which is the second state of its symmetry and as such is difficult to describe using standard single-reference methods. (C) 1997 Elsevier Science B.V.