  We study the pressure dependence of the melting mechanism of a surface free
Lennard-Jones crystal by constant pressure Monte Carlo simulation. The
difference between the overheating temperature($T_{OH}$) and the
thermodynamical melting point($T_M$) increase for increasing pressure. When
particles move into the repulsive part of the potential the properties at
$T_{OH}$ change. There is a crossover pressure where the volume jump becomes
pressure-independent. The overheating limit is pre-announced by thermal
excitation of big clusters of defects. The temperature zone where the system is
dominated by these big clusters of defects increases with increasing pressure.
Beyond the crossover pressure we find that excitation of defects and clusters
of them start at the same temperature scale related with $T_{OH}$.
