A particularly useful variant of the MCSCF method is the complete active space SCF (CASSCF) method. In this approach, an active space is defined, and the wavefunction is obtained by performing full configuration interaction (CI) within this active space. The latter, dynamical or non-static electron correlation, is generally treated with post-MCSCF methods, commonly referred to as multireference (MR) methods, such as MR coupled-cluster (MRCC) and configuration interaction (MRCI). Although these two MR approaches are highly accurate, their computational cost is significant. Another class of MR approaches is perturbation theory (MRPT), with the complete active space second-order perturbation theory (CASPT2) being one of the most well-known MRPTs. Note that the "static" electron correlation computed with the MCSCF method can contain contributions that should belong to the dynamical electron correlation in modern electron correlation theory. They are referred to as the "internal" dynamical electron correlation, and the rest, taken account by post-MCSCF methods, is referred to as the "external" dynamical correlation. These post-SCF treatments can also be applied to the single-configuration Hartree-Fock method. In particular, the CC approach can be extended to excited states through the equation-of-motion (EOM) formalism, 10 typically utilizing the CC singles and doubles (EOM-CCSD) method.