  Within Full Configuration Interaction Quantum Monte Carlo, we investigate how
the statistical error behaves as a function of the parameters which control the
stochastic sampling. We define the inefficiency as a measure of the statistical
error per particle sampling the space and per timestep and show there is a
sizeable parameter regime where this is minimised. We find that this
inefficiency increases sublinearly with Hilbert space size and can be reduced
by localising the canonical Hartree--Fock molecular orbitals, suggesting that
the choice of basis impacts the method beyond that of the sign problem.
