The multi-state CASPT2 spin-orbit method

TL;DR

This paper introduces a new multi-state CASPT2 spin-orbit method that accurately computes spin-orbit couplings without energy shifts, ensuring correct wave function characters for electronic structure calculations.

Contribution

The paper presents a novel two-step MS-CASPT2-SO method that improves accuracy and reliability in spin-orbit coupling calculations compared to previous approaches.

Findings

Does not use energy shifts in calculations

Guarantees correct wave function characters

Enhances accuracy of electronic structure predictions

Abstract

We propose the multi-state complete-active-space second-order perturbation theory spin-orbit method (MS-CASPT2-SO) for electronic structure calculations. It is a two-step spin-orbit coupling method that does not make use of energy shifts and that intrinsically guarantees the correct characters of the small space wave functions that are used to calculate the spin-orbit couplings, in contrast with previous two-step methods.