Relativistic corrections to the electric polarizability of compound systems

The widely used relation <(<alpha>)over tilde> = alpha (o) + Delta alpha for the electric polarizability, where Delta alpha is proportional to the r.m.s. charge radius of the system, is revisited within a simple model of two spinless particles bound electromagnetically. A complete set of the relativistic corrections to ceo following from the Breit Hamiltonian is obtained. It is shown that these corrections modify the result for a in comparison to that obtained with ao taken in the nonrelativistic approximation. We propose that the same situation can take place in the more complicated case of hadrons. Special attention is devoted to the correct definition of the center-of-mass coordinates which are found to be very important.