CASSCF with Extremely Large Active Spaces using the Adaptive Sampling Configuration Interaction Method

TL;DR

This paper introduces an advanced CASSCF approach using the Adaptive Sampling CI method, enabling accurate multiconfigurational calculations on much larger active spaces than traditional methods, with practical solutions to orbital optimization challenges.

Contribution

It demonstrates that ASCI can be integrated into CASSCF to handle active spaces with over 50 electrons and orbitals, significantly expanding the scope of strongly correlated system studies.

Findings

Achieved chemical accuracy in large active spaces (>50 electrons, >50 orbitals).

Demonstrated lack of polyradical character in large periacenes.

Compared ASCI-SCF results with heat-bath CI on iron porphyrin system.

Abstract

The complete active space self-consistent field (CASSCF) method is the principal approach employed for studying strongly correlated systems. However, exact CASSCF can only be performed on small active spaces of ~20 electrons in ~20 orbitals due to exponential growth in the computational cost. We show that employing the Adaptive Sampling Configuration Interaction (ASCI) method as an approximate Full CI solver in the active space allows CASSCF-like calculations within chemical accuracy (<1 kcal/mol for relative energies) in active spaces with more than ~50 active electrons in ~50 active orbitals, significantly increasing the sizes of systems amenable to accurate multiconfigurational treatment. The main challenge with using any selected CI-based approximate CASSCF is the orbital optimization problem; they tend to exhibit large numbers of local minima in orbital space due to their lack of invariance to active-active rotations (in addition to the local minima that exist in exact CASSCF). We highlight methods that can avoid spurious local extrema as a practical solution to the orbital optimization problem. We employ ASCI-SCF to demonstrate lack of polyradical character in moderately sized periacenes with up to 52 correlated electrons and compare against heat-bath CI on an iron porphyrin system with more than 40 correlated electrons.