Response Formalism within Full Configuration Interaction Quantum Monte Carlo: Static Properties and Electrical Response

We formulate a general, arbitrary-order stochastic response formalism within the full configuration interaction quantum Monte Carlo framework. This modified stochastic dynamic allows for the exact response properties of correlated multireference electronic systems to be systematically converged upon for systems far out of reach of traditional exact treatments. This requires a simultaneous coupled evolution of a response state alongside the zero-order state, which is shown to be stable, nontransient, and unbiased. We demonstrate this with application to the static dipole polarizability of molecular systems and, in doing so, resolve a discrepancy between restricted and unrestricted high-level coupled-cluster linear response results which were the high-accuracy benchmark in the literature.