Second-Order Complete Active Space Perturbation Theory (CASPT2) and <i>N</i>-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 <i>N</i>-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 (34<i>e</i>,34<i>o</i>).