Towards a field-free quantum Monte Carlo approach to polarizabilities of excited states: Application to the n = 2 hydrogen atom

Accurate calculation of excited state polarizabilities by diffusion Monte Carlo remains elusive. Here, the straight-forward approach based on the finite-field approximation is problematic due to the shifting of the physical nodes upon imposition of an external field. In this Letter we take the first steps towards developing a field-free approach to polarizabilities of excited states. We explain why the physical nodes introduce surprising sampling difficulties for some of the estimators and how to overcome these for the simplest case, namely the n = 2 hydrogen atom. We point out confusion in the literature in the definition and calculation of polarizabilities for this system.