  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.
