iCISCF: An Iterative Configuration Interaction-based Multiconfigurational Self-consistent Field Theory for Large Active Spaces

TL;DR

The paper introduces iCISCF, a new iterative configuration interaction-based method for multiconfigurational self-consistent field calculations, enabling efficient treatment of large active spaces with high accuracy.

Contribution

It presents a novel iCISCF approach combining configuration selection, orbital optimization, and perturbation theory for large active space systems.

Findings

iCISCF accurately approximates CASCI for large active spaces

iCISCF(2) provides high-accuracy CASSCF approximations

Demonstrated effectiveness on systems beyond traditional CASSCF capabilities

Abstract

An iterative configuration interaction (iCI)-based multiconfigurational self-consistent field (SCF) theory, iCISCF, is proposed to handle systems that require large complete active spaces (CAS). The success of iCISCF stems from three ingredients: (1) efficient selection of individual configuration state functions spanning the CAS, meanwhile maintaining full spin symmetry; (2) the use of Jacobi rotation for the optimization of active orbitals, in conjunction with a quasi-Newton algorithm for the core/active-virtual and core-active orbital rotations; (3) a second-order perturbative treatment of the residual space left over by the selection procedure (i.e., iCISCF(2)). Just like selected iCI being a very accurate approximation to CASCI, iCISCF(2) is a very accurate approximation to CASSCF. Several examples that go beyond the capability of CASSCF are taken as showcases to reveal the performances of iCISCF and iCISCF(2).