Approximations of density matrices in N-electron valence state second-order perturbation theory (NEVPT2). III. Large...

In multi-reference (MR) methods, addressing systems with large active spaces remains a challenge in the field. In Papers I and II of this series, we demonstrated that full rank N-electron valence state second-order perturbation theory (FR-NEVPT2) is a robust MR perturbation theory capable of computing strongly correlated systems with approximate density matrices. However, the previous FR-NEVPT2 implementation requires the computation and storage of fifth-order reduced density matrices (RDMs), limiting the usage of FR-NEVPT2 for systems with large active spaces. In the present work, as Paper III of the series, we report a new FR-NEVPT2 algorithm to handle systems with large active spaces. In the new algorithm, an approximate complete active space (CAS) self-consistent field (SCF) method, iterative configuration expansion (ICE) SCF, is employed to compute the reference wave functions for FR-NEVPT2. Then, the necessary Koopmans matrices of FR-NEVPT2 involving various RDMs are constructed using the intermediates designed by Kollmar et al. [J. Chem. Phys. 155, 234104 (2021)] to avoid storage bottlenecks. The performance of the new FR-NEVPT2 algorithm for systems with large active spaces is evaluated. Our results show that even with aggressive truncation parameters to truncate the ICE-SCF reference wave function, FR-NEVPT2 effectively recovers the missing static correlations of ICE-SCF. Several interesting systems with active spaces up to CAS(34,34) are studied using FR-NEVPT2 with ICE-SCF reference.