Some New Simplifications for General Form of Orbit–orbit Interaction Operator

The general tensorial form of the orbit–orbit interaction operator in the formalism of second quantization is presented. Such an expression is needed to calculate both diagonal and off–diagonal matrix elements with respect to configurations, in a uniform way. Some special cases are shown for which the orbit–orbit interaction operator reduces to simple form. The importance of different contributions to the Breit–Pauli hamiltonian is investigated in ground states and ionization potentials for Li isoelectronic sequency in the systematic way.