γ | is the projector onto the reference space, and Q = 1 -P is the projector onto the complementary space. The space of the reference states, which are typically obtained by solving state-averaged CASSCF (SA-CASSCF) equations, is represented by the symbol P. The Fock operator F can be either state-specific or state-averaged, and one can define several CASPT2 variants depending on the operator choice, as outlined below.