Near-Exact CASSCF-Level Geometry Optimization with a Large Active Space using Adaptive Sampling Configuration Interaction Self-Consistent Field Corrected with Second-Order Perturbation Theory (ASCI-SCF-PT2)

An accurate description of electron correlation is one the most challenging problems in quantum chemistry. The exact can be obtained by means full configuration interaction (FCI). A simple strategy for approximating FCI at a reduced computational cost selected CI (SCI), which diagonalizes Hamiltonian within only chosen space. Recovery contributions remaining configurations possible with second-order perturbation theory. Here, we apply adaptive sampling (ASCI) combined molecular orbital optimizations (ASCI-SCF) corrected theory (ASCI-SCF-PT2) geometry optimization implementing analytical nuclear gradient algorithm ASCI-PT2 Z-vector (Lagrangian) formalism. We demonstrate that phenalenyl radicals and anthracene, optimized geometries number unpaired electrons nearly CASSCF accuracy incorporating PT2 corrections extrapolating them. current algorithm’s utility optimizing equilibrium electronic structures six-ring-fused polycyclic aromatic hydrocarbons 4-periacene.