[en] The potential energy of the OCP in the fluid phase is known to great accuracy from Monte Carlo (MC) data obtained from very long simulations. For the crystalline phase, bcc or fcc, the MC simulations are equally accurate but can also be represented nearly analytically in the harmonic lattice (HL) approximation. We give here recent results for the anharmonic effects in the classical regime T >> T_p (where T_p is the ion plasma temperature) both exact and obtained in the HL approximation. We also give results for the harmonic potential energy in a quantum OCP crystal and discuss the importance of quantum effects for the fluid-lattice phase transition. (orig.)