The wave function of the reference state Ψ (0) is an eigenfunction of Ĥ(0) by construction (which does not necessarily have to be the ground state one), with an associated energy eigenvalue E . Note that in state-specific CASPT2, this energy does not correspond to the CASSCF one, but rather to the expectation value of the generalized Fock operator (this is just as in MP2, where the zeroth-order energy does note correspond to the Hartree-Fock one).