The CASPT2 method was initially developed and reported by Anderson et al. as one of the successful multiconfigurational perturbation theories based on non-relativistic or spin-free relativistic frameworks. In this method, static electron correlation is treated through a full CI calculation within the complete active space (CAS), achieved through either a CASCI or CASSCF calculation. Subsequently, dynamic electron correlation is incorporated using second-order perturbation theory, by introducing inactive and secondary orbital spaces, which were doubly occupied and unoccupied orbitals in the initial CASCI/CASSCF calculation.