Toward Accurate Spin-Orbit Splittings from Relativistic Multireference Electronic Structure Theory

Area of Science:

• Quantum Chemistry
• Relativistic Effects in Molecules
• Computational Chemistry

Background:

• Nonrelativistic electron correlation methods can be adapted for relativistic effects using molecular spinor integrals.
• Relativistic multireference correlation methods are less explored, with uncertain benefits of perturbative treatments.

Purpose of the Study:

• To implement state-averaged four-component relativistic multireference perturbation theories to second and third order.
• To assess the performance of these new methods for relativistic electron correlation.

Main Methods:

• Implementation of state-averaged four-component relativistic multireference perturbation theories to second and third order.
• Utilizing the driven similarity renormalization group (DSRG) method.
• Application to p-block elements.

Main Results:

• The new methods (4c-SA-DSRG-MRPT2 and 4c-SA-DSRG-MRPT3) significantly improve spin-orbit splittings and potential energy surfaces compared to 4c-CASSCF and 4c-CASPT2.
• The methods show applicability over a wide flow parameter range.
• Third-order calculations offer improved error statistics and reduced flow parameter sensitivity over second-order.

Conclusions:

• Dynamical correlation in relativistic multireference perturbation theories offers significant improvements.
• The DSRG-based methods provide a robust and accurate approach for relativistic electronic structure calculations.
• Further development in relativistic multireference correlation methods is warranted.