  Following Dirac's ideas concerning the quantization of constrained systems,
we suggest to replace the free centre of mass Hamiltonian H_{CM} by another
operator which commutes with all the elements of the algebra generated via the
commutation relations by H_{CM} and the constraints which fix the centre of
mass position. We show that the new Hamiltonian is a multiple of the identity
operator and, as a result, its unique effect is to raise the internal energy
levels by a constant amount.
