Excited states by quantum Monte Carlo methods: Imaginary time evolution with projection operators

Abstract
We present a Monte Carlo algorithm suitable for the calculation of excited state energies of multidimensional quantum systems. Energies are extracted from a maximum entropy analysis of the imaginary time evolution of a state prepared by application of a projection operator on an initial wave function. The imaginary time evolution is computed with a pure diffusion Monte Carlo algorithm. The method is demonstrated on a harmonic oscillator and several Morse oscillator test problems.