Electrodynamics of finite-size particles with arbitrary spin

We construct the most general Hamiltonian for the electromagnetic interaction of the finite-size particle-like nucleus with arbitrary spin, magnetic dipole, and electric quadrupole moments. It includes all the terms, which are important for obtaining atomic energy levels up to the order alpha(6). The result is verified against spin s = 0, 1/2, and 1 cases, where the Foldy-Wouthuysen transformation is performed of the corresponding relativistic equation.