Excitations with Diffusion Monte Carlo: a bottom-up approach.

The applications of diffusion Monte Carlo (DMC) have been largely focused on properties of the ground state. As a result, high quality methods to obtain and optimize the ground-state trial wavefunctions are available. However, since much of the phenomena in Solid State Physics is governed by low energy excitations, attempts to extend DMC for the calculations of excited states properties are also documented in the literature. Most of those attempts focus on the direct calculation of excitations. In this talk we will discuss an alternative approach: how to extract excited state properties as an observable of the many-body ground state evaluated within a standard DMC method. The theory will be tested in a model system and compared with exact solutions. The potential limitations of this indirect approach for large systems of interest for real material problems will be discussed.