On the Breit interaction in an explicitly correlated variational Dirac-Coulomb framework

The Breit interaction is implemented in the no-pair variational Dirac-Coulomb (DC) framework using an explicitly correlated Gaussian basis reported in the previous paper [Paper I: P. Jeszenszki, D. Ferenc, and E. M\'atyus (2022)]. Both a perturbative and a fully variational inclusion of the Breit term is considered. The no-pair DC plus perturbative Breit as well as the no-pair Dirac-Coulomb-Breit energies are compared with perturbation theory results including the Breit-Pauli Hamiltonian and leading-order non-radiative quantum electrodynamics corrections for low $Z$ values. Possible reasons for the observed deviations are discussed.