When one conducts simulations on melting of nanowires, one observes the potential energy of the system. In the beginning the potential energy increases almost linearly with increase in the temperature but at the melting point there is a sudden rise in the potential energy. Is this rise due to the phase change, if so how can it be explained.

After the melting point the potential energy once again increases linearly, so what is the reason for this?

Thank you.