  New model describing the pressure effect on the melting temperature is
proposed by using four assumptions. One, the average wavelength of the phonon
vibration at the Debye temperature corresponds to the length of the unit cell.
Two, the phonon vibration at the melting temperature is in self-resonance with
the lattice vibration of the surface atomic/molecular layer. Three, the phonon
wavelength ratio of the Debye and the melting temperature does not be affected
by the pressure. Four the pressure reduces the anharmonic part of the
vibration. The relevant equations are derived and tested against the
experiments of sodium with positive result.
