Based on the Debye model, the free energy of a nonideal electron-ion plasma is calculated for interaction parameters 0 < Y < Ye below the critical solid state value Ye (y = Ze 2 n 1 / 3 /KTis the ratio of mean Coulomb interaction energy to thermal energy), which takes into account the energy eigenvalues of (i) the thermal translational particle motions, (ii) the random collective electron and ion motions, and (iii) the static Coulomb interaction energy of the electrons and ions in their oscillatory equilibrium positions. From this physical model, the interaction part of the free energy is derived, which consists of a quasi-lattice energy, depending on the interaction parameter Y, and the free energies of the quantized electron and ion oscillations (long-range interactions). Depending on the degree of ordering, the Madelung 'constant' of the plasma is oc(y) = ii. for Y ~ 1, oc(y) :::; ii. for Y > 1 and oc(y) oc yl12 for y <{ 1, where ii. ~ 1 is a constant. The free energy of the high frequency plasmons (electron oscillations) is shown to be very small for y > 1, whereas the free energy of the low frequency plasmons (ion oscillations) is shown to be significant for y > 1, i.e. for proper nonideal conditions. From the general formula for the free interaction energy !!F of the plasma for