  This paper deals with the sign problem in full configuration-interaction
quantum Monte Carlo.
  After putting this article on the arxiv, it was pointed out to us that our
argument applies to a number of model Hamiltonians we considered numerically
but not to the most generic case. Please see arxiv: 1209.3044, where we have a
corrected comprehensive discussion of the necessary and sufficient conditions
for when the sign problem for a given Hamiltonian (and basis) differ between
first and second quantization.
