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 set 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 c ^{1}A_{1} state, which is the
second state of its symmetry and as such is difficult to
describe using standard single-reference methods.