Capturing the electron correlation as fully as possible is essential to accurately describe the complex electronic structures of atoms and molecules. The electron correlation is formally split into "static" and "dynamic" components, which result from near-degenerate configuration state functions (CSFs) and instantaneous repulsion of electrons, respectively. The static electron correlation is described using the multiconfiguration self-consistent field (MC-SCF) method, in which the wavefunction is expressed as a linear combination of the selected CSFs. A particularly useful variant of the MCSCF method is the complete active space SCF (CASSCF) method. We define an active space and express the wavefunction by performing the full configuration interaction within the active space. However, the static electron correlation is not sufficient to describe complex electronic structures; we also require the dynamic electron correlation, which is treated with post-MCSCF methods. These are typically referred to as multireference (MR) methods and include MR coupled-cluster and MR configuration interaction (MRCI) approaches. Although both of these MR approaches are accurate, their computational cost is high. Another type of MR approach is MR perturbation theory (MRPT). Owing to recent developments of analytic derivative theories, MRPT is a practical choice for accurately determining molecular structures or even performing molecular dynamics (MD) simulations.