  Strong pairing correlations are responsible for superconductivity and
off-diagonal long range order in the two-particle density matrix. The
antisymmetrized geminal power wave function was championed many years ago as
the simplest model that can provide a reasonable qualitative description for
these correlations without breaking number symmetry. The fact remains, however,
that the antisymmetrized geminal power is not generally quantitatively accurate
in all correlation regimes. In this work, we discuss how we might use this wave
function as a reference state for a more sophisticated correlation technique
such as configuration interaction, coupled cluster theory, or the random phase
approximation.
