A coupled oscillator model and a collective description given for electron gases in foregoing papers have been applied to an electron-ion plasma at high density. An expression for the correlation energy of a plasma has been given by using Wick's expansion of chronological operators. After separating the plasma mode from the electron-ion interaction completely, a coupling parameter of the interaction between individual electrons and plasma phonons has been determined. Froelick's coupling parameter in his theory of the superconductivity has been confirmed at the high density limit.
