  Using the homogeneous electron gas (HEG) as a model, we investigate the
sources of error in the `initiator' adaptation to Full Configuration
Interaction Quantum Monte Carlo (i-FCIQMC), with a view to accelerating
convergence. In particular we find that the fixed shift phase, where the walker
number is allowed to grow slowly, can be used to effectively assess stochastic
and initiator error. Using this approach we provide simple explanations for the
internal parameters of an i-FCIQMC simulation. We exploit the consistent basis
sets and adjustable correlation strength of the HEG to analyze properties of
the algorithm, and present finite basis benchmark energies for N=14 over a
range of densities $0.5 \leq r_s \leq 5.0$ a.u. A \emph{single-point
extrapolation} scheme is introduced to produce complete basis energies for 14,
38 and 54 electrons. It is empirically found that, in the weakly correlated
regime, the computational cost scales linearly with the plane wave basis set
size, which is justifiable on physical grounds. We expect the fixed shift
strategy to reduce the computational cost of many \iFCIQMC calculations of
weakly correlated systems. In addition, we provide benchmarks for the electron
gas, to be used by other quantum chemical methods in exploring periodic solid
state systems.
