As for CAS, GAS-CI wave functions are preselected by the user, through chemical (and/or physical) considerations, and a careful choice of active orbitals and electrons. The active orbitals are subsequently partitioned in a number of active subspaces. The nature, size, and number of these subspaces largely depend on the investigated systems, and generally are chosen according to the type of electron correlation that one wants to target or exclude from the CI space. The examples discussed in the Application section or in Reference 27 can be used as guidelines to the strategic choice of GAS subspaces.