Gaunt and Breit Two-electron contributions to Mean-field Transformations and Fine Structure Splitting

TL;DR

This paper develops a relativistic mean-field computational framework incorporating two-electron Gaunt and Breit interactions, demonstrating their increasing significance for heavy elements' electronic structure calculations.

Contribution

It introduces an exact two-component mean-field approach including all two-electron relativistic contributions, advancing accuracy in heavy-element electronic structure modeling.

Findings

Growing contributions of Gaunt and Breit integrals with increasing atomic number.

The framework enables more accurate relativistic calculations for heavy elements.

It lays the groundwork for future theoretical developments in relativistic quantum chemistry.

Abstract

Materials utilized by novel energy systems are often studied using weakly correlated mean-field theories. However, if these systems incorporate heavy elements, relativistic effects must be included. Therefore a Kramers unrestricted Coupled Cluster with singles and doubles excitation formalism within a molecular mean-field Exact Two-Component framework (X2C-mmf) using a four-component Dirac-Hartree-Fock (DHF) reference state is presented. The exact X2C-mmf transformed normal-order Hamiltonian incorporates all one-electron and two-electron (2e) contributions from the Coulomb, Gaunt, and Breit operators and is used with the Equation of Motion method to calculate the excitation energies of the alkali group of elements. Using this framework, the effects of 2e Gaunt and Breit integrals are studied. Results demonstrate growing contributions from these integrals to the generated X2C-mmf mean-fields and electronic fine structure calculations with increasing atomic number. Overall, this paper outlines the method and effect of a higher level of accuracy within the X2C-mmf approach and lays the foundation for future theoretical development of relativistic calculations within this framework.