A Perturbative Super-CI Approach for orbital optimization in Two-Component relativistic CASSCF

TL;DR

This paper introduces a new perturbative Super-CI approach for orbital optimization in two-component relativistic CASSCF, effectively incorporating spin--orbit coupling and relativistic effects for improved accuracy in multireference quantum chemistry.

Contribution

The paper develops the Super-CIPT method for 2C-CASSCF, enabling robust orbital optimization with relativistic effects and strong SOC, outperforming traditional methods.

Findings

Super-CIPT shows robust convergence and applicability to systems with strong SOC.

Inclusion of Gaunt or Breit terms reduces errors below 2% for halogens.

2C-CASSCF outperforms one-component CASSCF in spin-orbit splitting calculations.

Abstract

In this work, we develop a new orbital optimization approach, perturbative Super-CI (Super-CIPT), for the two-component complete active space self-consistent field (2C-CASSCF) method. By variationally optimizing spinor orbitals and consistently incorporating spin--orbit coupling (SOC) at the orbital level, the 2C-CASSCF method enables a simultaneous treatment of relativistic effects and static correlation. The Super-CIPT approach demonstrates robust convergence behavior and is applicable to systems under strong SOC. The inclusion of Gaunt or Breit term via the atomic mean field approximation yields the most accurate results, with errors dropping below 2% for halogens. We systematically assess the performance of 2C-CASSCF on spin-orbit splittings (SOSs) of selected p-block elements. Results show that 2C-CASSCF outperforms conventional one-component (1C) CASSCF. This work establishes 2C-CASSCF with Super-CIPT as a reliable and efficient approach for multireference relativistic quantum chemistry.