Accounting for the Breit interaction in relativistic effective core potential calculations of actinides

The accuracy of accounting for the Breit effects in calculations of atoms and molecules in the framework of the two-component relativistic effective core potential (RECP) formalism is analysed theoretically and illustrated in calculations. Contributions of the Breit interaction between different core and valence shells are studied in the framework of the four-component SCF approximation for uranium and plutonium. It is shown that two-electron Breit effects between valence electrons can be neglected for the calculation of valence energies in systems containing actinides with 'chemical accuracy' (1 kcal mol −1 or 350 cm −1 ), whereas large core–core and core–valence Breit contributions can be efficiently described by one-electron RECP operators. Different versions of generalized RECPs with the Breit interaction taken into account are constructed for uranium and plutonium and compared for accuracy with the corresponding all-electron four-component calculations.