  By using molecular dynamics simulation, formation mechanisms of amorphous
carbon in particular sp${}^3$ rich structure was researched. The problem that
reactive empirical bond order potential cannot represent amorphous carbon
properly was cleared in the transition process from graphite to diamond by high
pressure and the deposition process of amorphous carbon thin films. Moreover,
the new potential model which is based on electron distribution simplified as a
point charge was developed by using downfolding method. As a result, the
molecular dynamics simulation with the new potential could demonstrate the
transition from graphite to diamond at the pressure of 15 GPa corresponding to
experiment and the deposition of sp${}^3$ rich amorphous carbon.
