A hybrid approach to extending selected configuration interaction and full configuration interaction quantum Monte Carlo

Understand

We present an approach to combining selected configuration interaction (SCI) and initiator full configuration interaction quantum Monte Carlo (i-FCIQMC).

• In the current i-FCIQMC scheme, the space of initiators is chosen dynamically by a population threshold.
• Here, we instead choose initiators as the selected space (V) from a prior SCI calculation, allowing substantially larger initiator spaces for a given walker population.
• While SCI+PT2 adds a perturbative correction in the first-order interacting space (FOIS) beyond V, the approach presented here allows a variational calculation in the same space, and a perturbative correction in the second-order interacting space.