As illustrated below by numerical examples, the application of this procedure to an open-shell reference results in an erratic behavior, which can be attributed to an unbalanced truncation of the α and β virtual orbital spaces. To solve this problem, we developed an algorithm for separating the virtual β space into a subspace matching the orbitals which are singly occupied in the high-spin reference and the rest. This procedure effectively determines the singly occupied subspace and uses the same strategy as proposed in Ref. . We note that the issue of imballanced tructation of α and β spaces does not appear in a fully spinadapted formulation .