An excited-state approach within full configuration interaction quantum Monte Carlo.

We present a new approach to calculate excited states with the full configuration interaction quantum Monte Carlo (FCIQMC) method. The approach uses a Gram-Schmidt procedure, instantaneously applied to the stochastically evolving distributions of walkers, to orthogonalize higher energy states against lower energy ones. It can thus be used to study several of the lowest-energy states of a system within the same symmetry. This additional step is particularly simple and computationally inexpensive, requiring only a small change to the underlying FCIQMC algorithm. No trial wave functions or partitioning of the space is needed. The approach should allow excited states to be studied for systems similar to those accessible to the ground-state method due to a comparable computational cost. As a first application, we consider the carbon dimer in basis sets up to quadruple-zeta quality and compare to existing results where available.N.S.B. gratefully acknowledges Trinity College, Cambridge for funding. G.H.B. gratefully acknowledges the Royal Society for funding via a university research fellowship. This work has been supported by the EPSRC under grant no. EP/J003867/1.This is the accepted manuscript. The final version is available at http://scitation.aip.org/content/aip/journal/jcp/143/13/10.1063/1.4932595