postCASSCF

PostCASSCF refers to a family of electronic-structure methods that are applied after a Complete Active Space Self-Consistent Field (CASSCF) calculation to recover dynamic electron correlation and to describe excited states more reliably. CASSCF handles static correlation within a chosen active space, but it neglects much of the dynamic correlation present in real systems; postCASSCF methods aim to correct this deficiency and provide improved energetics and potential energy surfaces.

The most widely used postCASSCF techniques are CASPT2 (Complete Active Space Second-Order Perturbation Theory) and NEVPT2 (N-electron Valence State Perturbation Theory). CASPT2 treats dynamic correlation as a second-order perturbation correction to the CASSCF reference and often requires level shifts, such as an IPEA shift, to mitigate intruder-state problems. NEVPT2 offers a perturbative correction that is intruder-free and generally size-consistent, providing robust results across a range of systems. Both methods depend on the quality of the underlying CASSCF active space and can be applied to ground and excited states, though their accuracy hinges on the suitability of the active space and state-averaging choices.

Beyond perturbation theory, multi-reference configuration interaction (MRCI) methods can be employed on top of CASSCF to capture dynamic correlation, at greater computational cost. Challenges common to postCASSCF approaches include selecting an appropriate active space, managing intruder states, and ensuring balanced treatment of different electronic states. These methods are widely used for systems with strong static correlation, such as bond dissociation, transition-metal complexes, and photochemical processes, where single-reference methods fail to provide reliable results.