A second-order multi-reference quasiparticle-based perturbation theory

Assessment of State-Averaged Driven Similarity Renormalization Group on Vertical Excitation Energies: Optimal Flow Parameters and Applications to Nucleobases

“…The wave function is then made flexible by allowing electrons excited from occupied to virtual orbitals (dynamical correlation). In this MR framework, numerous methods have been developed, including MR perturbation theory (PT), − MR-ADC, MRCI, − and many flavors of MRCC along with the LR/EOM generalizations. − In particular, second-order MRPTs (MRPT2s) are more commonly applied due to the favorable accuracy–cost balance. The second-order complete-active-space (CAS) PT (CASPT2) and n -electron valence state PT (NEVPT2) are two of the most well-known MRPT2s.…”