Because the αα and ββ blocks of the density matrix were diagonalized separately, the there is no guarantee of direct correspondence between the α and β NOs. Moreover, the numerical values of the α and β occupancies are different. Sometimes the α and β orbitals are ordered in different manner. This introduces an imbalance between the frozen α and β orbital subspaces, causing spin contamination. ). Just as in the closed-shell case, OSFNO performs better than freezing the same number of the canonical orbitals (Figure ). The mean error for a typical total population of 99% for the methylene series are 0.02-0.03 eV (130 cm -1 for open-shell singlet-triplet gap, 200 cm -1 for a gap between closed-shell singlet and the triplet states).