  The anharmonicity resulted from the intrinsic phonon interaction is neglected
by quasiharmonic approximation. Although the intensive researches about
anharmonicity have been done, up to now the free energy contributed by the
anharmonicity is still difficult to calculate. Here we put forward a new method
that can well include the anharmonicity. We introduce the implicit temperature
dependence of effective frequency by volume modification. The quasiharmonic
approximation becomes a special case in our method corresponding to non volume
modification. Although our method is simple and only a constant need to
determine, the anharmonicity is well included. Thermodynamic properties of MgO
predicted with our method are excellent consistent with the experiment results
at very wide temperature range. We also believe that our method will be helpful
to reveal the characteristic of anharmonicity and intrinsic phonon interaction.
