Complete active space perturbation theory
Complete active space perturbation theory (CASPTn) is a multireference electron correlation method for computational investigation of molecular systems, especially for those with heavy atoms such as transition metals, lanthanides, and actinides. It can be used, for instance, to describe electronic states of a system, when single reference methods and density functional theory cannot be used, and for heavy atom systems for which quasi-relativistic approaches are not appropriate. Although perturbation methods such as CASPTn are successful in describing the molecular systems, they still need a Hartree-Fock wavefunction to provide a valid starting point. The perturbation theories cannot reach convergence if the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are degenerate. Therefore, the CASPTn method is usually used in conjunction with the Multi-configurational self-consistent field method (MCSCF) to avoid near-degeneracy correlation effects. (Wikipedia).
