Crossings between potential energy surfaces (PESs) of different states play an important role in photochemistry. Typically, those between the same-spin states and spatial symmetries are referred to as conical intersections (CIs) and are responsible for non-radiative transitions between states. In particular, minimum energy CIs (MECIs) act as an efficient "funnel" around the CI region, with higher states efficiently and non-radiatively decaying to lower states. Because the electronic structure at crossing points is considerably more complex than the one at a minimum energy region of a PES, it is necessary to select a method that appropriately describes the electron correlation. One useful and well-known approach is the multiconfiguration self-consistent field (MCSCF) method, in particular the complte active space (CASSCF) version. Electron correlation described using MCSCF and CASSCF is typically referred to as "static" or "non-dynamic" electron correlation, which is attributed to near-degenerate configuration state functions (CSFs) or determinants, whereas "dynamic" electron correlation, which results from instantaneous repulsion of electrons, is missing. A large portion of the dynamic electron correlation can be recovered using post-MCSCF treatments, which are typically referred to as multireference (MR) methods, such as MR coupled-cluster and configuration interaction 7 approaches. However, in terms of computational cost vs accuracy, MR perturbation theory (MRPT) may be the most balanced approach. In particular, when one wants to determine minimum energy structures or perform molecular dynamics (MD) simulations, MRPT is a practical choice for mediumsized systems among the abovementioned MR methods, because these calculations are more time-consuming than a single-point energy evaluation.