where Ψ I is a EOM target state, R and L are general excitation operators, and Φ 0 is the reference determinant, which defines the separation between occupied and virtual orbital spaces. The equations for the amplitudes of the EOM operators are derived variationally, leading to a CI-like eigenproblem . In contrast to CI, the EOM theory employs a (non-Hermitian) similarity-transformed Hamiltonian: