The classical model of the point particle with spin (point structural par t
ide or p.s.p.) is applied to a system of two particles interacting electrom
agnetically. The interaction energy of the subparticles of one p.s.p. with
those of the other is described by Darwin's Lagrangian obtaining the energy
of the system and the classical equations of motion for each p.s.p. In the
expression of the energy, the terms of the spin-orbit interaction between
particles appear, as well as others which are found in Breit's quantum Hami
ltonian.