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)

TL;DR

This paper introduces an efficient quantum chemistry method combining adaptive sampling CI with SCF and PT2 corrections, achieving near-CASSCF accuracy for geometry optimization of complex molecules.

Contribution

It develops a new algorithm integrating ASCI-SCF with PT2 corrections and analytical gradients, enabling accurate geometry optimization at reduced computational cost.

Findings

Achieves near-CASSCF accuracy for phenalenyl radicals and anthracene geometries.

Successfully optimizes structures of polycyclic aromatic hydrocarbons.

Demonstrates computational efficiency and accuracy improvements over traditional methods.

Abstract

An accurate description of electron correlation is one of the most challenging problems in quantum chemistry. The exact electron correlation can be obtained by means of full configuration interaction (FCI). A simple strategy for approximating FCI at a reduced computational cost is selected CI (SCI), which diagonalizes the Hamiltonian within only the chosen configuration space. Recovery of the contributions of the remaining configurations is possible with second-order perturbation theory. Here, we apply adaptive sampling configuration interaction (ASCI) combined with molecular orbital optimizations (ASCI-SCF) corrected with second-order perturbation theory (ASCI-SCF-PT2) for geometry optimization by implementing the analytical nuclear gradient algorithm for ASCI-PT2 with the Z-vector (Lagrangian) formalism. We demonstrate that for phenalenyl radicals and anthracene, optimized geometries and the number of unpaired electrons can be obtained at nearly the CASSCF accuracy by incorporating PT2 corrections and extrapolating them. We demonstrate the current algorithm's utility for optimizing the equilibrium geometries and electronic structures of 6-ring-fused polycyclic aromatic hydrocarbons and 4-periacene.