Second-Order Complete Active Space Perturbation Theory (CASPT2) and N‑Electron Valence State Perturbation Theory (NEVPT2) Based on Adaptive Sampling Configuration Interaction Self-Consistent Field (ASCI-SCF)

We have developed the second-order complete active space perturbation theory (CASPT2) and N-electron valence state perturbation theory (NEVPT2) based on the adaptive sampling configuration interaction self-consistent field (ASCI-SCF) reference function. Our method directly calculates the intermediate matrices needed for the CASPT2 and NEVPT2 calculations to reduce the memory required for storing the four-particle reduced density matrix (4RDM). To demonstrate the method’s applicability, we evaluated the singlet–triplet gaps in several polyacenes and the potential energy curves of chromium dimer, in which the largest active space we tested was (34e,34o).