The explicit volume dependence of the vibrational part of the free energy is derived in terms of the first two moments of the mode gamma distribution and general characteristics of the lattice potential. Contrary to what is generally assumed, the approximation ⟨&ggr;2⟩=⟨&ggr;⟩2 is inadequate for compounds in the NaCl structure and the distribution in mode gamma values has a significant contribution to the volume dependence of the Gr¿neisen parameter.