In FCIQMC at any point of the dynamics the spawning step is responsible for the stochastic propagation of walkers into the CI space, starting from occupied determinants. Thus, if we assume that our starting determinant is allowed by GAS constraints, only the spawning step has to be modified to ensure that all spawned determinants are GAS allowed. The algorithmic details to realize a GAS-PCHB excitation generator are described in this section.