Wmc. Foulkes et al., Symmetry constraints and variational principles in diffusion quantum MonteCarlo calculations of excited-state energies, PHYS REV B, 60(7), 1999, pp. 4558-4570
Fixed-node diffusion Monte Carlo (DMC) is a stochastic algorithm for findin
g the lowest energy many-fermion wave function with the same nodal surface
as a chosen trial function. It has proved itself among the most accurate me
thods available for calculating many-electron,ground states, and is one of
the few approaches that can be applied to systems large enough to act as re
alistic models of solids. In attempts to use fixed-node DMC for excited-sta
te calculations, it has often been assumed that the DMC energy must be grea
ter than or equal to the energy of the lowest exact eigenfunction with the
same symmetry as the trial function. We show that this assumption is not ju
stified unless the trial function transforms according to a one-dimensional
irreducible representation of the symmetry group of the Hamiltonian. If th
e trial function transforms according to a multidimensional irreducible rep
resentation, corresponding to a degenerate energy level, the DMC energy may
lie below the energy of the lowest eigenstate of that symmetry. Weaker var
iational bounds may then be obtained by choosing trial functions transformi
ng according to one-dimensional irreducible representations of subgroups of
the full symmetry group. [S0163-1829(99)09331-5].