Beyond Brillouin's Theorem: On the Importance of Single Excitations in Jastrow-Correlated Wave Functions

The use of CIPSI (configuration interaction using a perturbative selection made iteratively) nodes in Quantum Monte Carlo calculations has been successfully applied in the past decade, involving optimization of Jastrow, CI, and MO parameters. In this work, we demonstrate that reoptimizing the CI coefficients in the presence of a Jastrow factor results in a substantial increase in the contribution of singly excited configurations, in clear contrast to Brillouin's theorem for conventional CI expansions. We analyze the effect of these singly excited configurations on CIPSI-based wave functions for water, ethene, and formaldehyde. Furthermore, we show that the inclusion of singles alone significantly reduces the node-location error in diffusion Monte Carlo calculations across a range of molecular systems.