  In this communication, we propose a method for obtaining isolated excited
states within the Full Configuration Interaction Quantum Monte Carlo framework.
This method allows for stable sampling with respect to collapse to lower energy
states and requires no uncontrolled approximations. In contrast with most
previous methods to extract excited state information from Quantum Monte Carlo
methods, this results from a modification to the underlying propagator, and
does not require explicit orthogonalization, analytic continuation, transient
estimators or restriction of the Hilbert space via a trial wavefunction.
Furthermore, we show that the propagator can directly yield frequency-domain
correlation functions and spectral functions such as the density of states
which are difficult to obtain within a traditional Quantum Monte Carlo
framework. We demonstrate this approach with pilot applications to the neon
atom and beryllium dimer.
