At the beginning of the simulation the shift parameter, S, is kept constant (generally initialized to a small real number or equal to zero). This allows the walker population to grow until the target population is reached. Once the target population is reached, the spawn, annihilation, and death steps are repeated with a shift parameter that is varied such that the target population stays constant. The calculation is carried in stationary conditions to collect sufficient data points for a satisfactory statistical analyis. The stochastic error of FCIQMC can be greatly reduced by using the semi-stochastic method. After stochastic propagation of equation 6, the n core most occupied determinants, representing the core space of the evolving CI wave function, are identified and the full Hamiltonian matrix, H core , for these configurations is constructed. The dynamics is propagated deterministically inside the core space and stochastically outside.