For most chemical systems, the full CI expasion in the active space is unnecessarily large, since CAS wave functions are general sparse, mostly containing "deadwood ", that is electronic configurations with vanishingly small CI amplitudes. Various methods exist that attempt to exclude deadwood from the CI optimization step, either via a sparse wave function representation, via a user preselection of truncated CI expansions, via an on the fly selection of the important electronic configurations, or by exploiting redundancies within the many-body wave function.