The spin-adaptation of single-reference quantum chemical methods for excited states of
open-shell systems has been nontrivial. The primary reason is that the configuration space,
generated by a truncated rank of excitations from only one component of a reference
multiplet, is spin-incomplete. Those “missing” configurations are of higher ranks and can, in
principle, be recaptured by a particular class of excitation operators. However, the resulting
formalisms are then quite involved and there are situations [eg, time-dependent density …