For delocalized molecular orbital bases, double excitations are usually more important and numerous than single excitations. Single excitations often feature vanishingly small matrix elements, for they (partially) fulfill the Brillouin theorem in its single-reference or generalized form. Therefore, most effort in the community has gone into improving the double excitation generation. However, in localized MO bases single excitations become as (or more) important than double excitations, capturing mean field effects via the CI-expansion.