Spin-generator coordinate method for electronic structure

We present a new application of the generator coordinate method (GCM) as an electronic structure method for strong electron correlation in molecular systems. We identify spin fluctuations as an important generator coordinate responsible for strong static electron correlation that is associated with bond-breaking processes. Spin-constrained unrestricted HF (c-UHF) states are used to define a manifold of basis states for the Hill-Wheeler equations, which are discretized and solved as a non-orthogonal configuration interaction expansion. The method was tested on two-electron systems that are dominated by static and/or dynamic correlations. In a minimal basis set for H2, the resulting GCM quickly captures the ground-state full configuration interaction energy with just a few c-UHF states, whereas second-order perturbation theory on top of the GCM is needed to recover over 90% of the correlation energy in the cc-pVDZ basis set.