The original FCIQMC algorithm suffers from a sign-problem in its application to most systems including ab-inito ones. When the number of walkers is below a certain threshold, called the annihilation plateau, the sampled wave function does not have a stable signstructure and is dominated by sign-incoherent noise. The annihilation plateau depends on the system under study and is typically a non-negligible fraction of the overall size of the Hilbert space. This means that one needs a minimum number of walkers that scales exponentially with the number of electrons and the number of orbitals. The problem is largely overcome by applying the initiator approximation, i-FCIQMC, which obviates the annihilation plateau and allows a stable simulation using small numbers of walkers. In i-FCIQMC, a walker is classified as an initiator if the determinant it is residing on has a population above a chosen threshold n add (usually set to three). Only initiators are allowed to spawn onto empty determinants, while non-initiators can only spawn onto other occupied determinants.