where a suitable choice of p gen (i, j) ∝ |K ij | leads to a more stable acceptance probability. The suggestion of new determinants is called the excitation generation step, and it is at the heart of an efficient implementation of FCIQMC. The excitation generation and spawning steps are "embarassingly parallel" processes. Depending on the signs of the parent determinants, D i , and of the corresponding K ij it is possible that spawns to D j with different signs arise from different D i determinants, which is a manifestation of the sign-problem within the FCIQMC algorithm. To partially control the sign problem spawns of opposite sign to the same determinant are summed at each time-step. This process represents the annihilation step.