A more precise expression for Δ𝐸 ST can be obtained by configuration interaction singles (CIS). While CIS adds some correlation for the excited states, it is necessary to add additional excitations beyonds singles to capture the inverted Δ𝐸 ST . Fortunately, the most important excitations have already been identified in the literature long ago. Singlet-triplet inversion occurs also for the ground state, where several violations of Hund's rule are well known, for example for bond-equalized cyclobutadiene at the D 4h geometry. Borden and Davidson explained this effect in terms of dynamic spin polarization (DSP), in which the electrons of a pair of disjoint non-bonding molecular orbitals (NBMOs, Figure ) experience stronger electron correlation effects in the singlet state as compared to the triplet state (Figure ). For a pedagogic introduction to the topic, the reader is referred to an excellent article by Karafiloglou. As outlined by Kollmar and Staemmler, 3 as well as Malrieu and co-workers, an alternative view of spin polarization is through admixture of excited configurations into the electronic wavefunction of the ground state (Figure ). This is shown in Figure for the case of static spin polarization in the allyl radical. The DSP effect for the S 1 excited state can correspondingly be described by admixture of doubly excited configurations, singly excited with respect to the S 1 state, and doubly excited with respect to the S 0 ground state (Figure ).