  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.
