Towards accurate spin-orbit splittings from relativistic multireference electronic structure theory

TL;DR

This paper introduces new relativistic multireference perturbation theories based on the driven similarity renormalization group, significantly improving spin-orbit splitting predictions in p-block elements compared to previous methods.

Contribution

First implementation of state-averaged four- and three-component multireference perturbation theories using DSRG, enhancing accuracy of relativistic electronic structure calculations.

Findings

Dynamical correlation improves spin-orbit splittings.

Methods are applicable across a range of flow parameters.

Systematic accuracy improvement from second to third order.

Abstract

Most nonrelativistic electron correlation methods can be adapted to account for relativistic effects, as long as the relativistic molecular spinor integrals are available, from either a four-, two-, or one-component mean-field calculation. However, relativistic multireference correlation methods remain a relatively unexplored area, with mixed evidence regarding the improvements brought by perturbative treatments. We report, for the first time, the implementation of state-averaged four-component multireference perturbation theories to second and third order based on the driven similarity renormalization group (DSRG). With our methods, named 4c-SA-DSRG-MRPT2 and 3, we find that the dynamical correlation included on top of 4c-CASSCF references can significantly improve the spin-orbit splittings in p-block elements and potential energy surfaces when compared to 4c-CASSCF and 4c-CASPT2 results. We further show that 4c-DSRG-MRPT2 and 3 are applicable to these systems over a wide range of the flow parameter, with systematic improvement from second to third order in terms of both improved error statistics and reduced sensitivity with respect to the flow parameter.