Helleo Yahia,

There are many reasons to get positive energy for your system in MD simulation. It could be your initial configuration is not correct so some atoms overlap. People usually do routine check before running MD simulation. So I would recomend first to check your initial configuration and then minimize it so you can avoid overlapping atoms.

Thank Ahmed very much, I will do that.

Hello,

As Ahmed said  it is probably an unreasonable geometry in your initial conditions. Check if you have more than one atom in the same place. If you are using periodic boundary conditions, maybe some atoms are overlapping with the periodic image of another one.

Good luck. 

Thank you very mcuh Maximiliano for your help, I try to check that.

What other potentials did you use besides the Buckingham-potential? Normally the bare Buckingham-potential itself contributes positively to the energy. Coulomb- and VdW interactions give a negative contribution.

Thank you very much H. P. Huinink.

I used Buckingham (Coulomb, VDW)potential. Now the problem is solved, because there was a disorder of parameters only, but I found a constant temperature and total energy, it means no fluctuations.This will not be acceptable because when a plot the curve of total energy VS volume or parameter of unit cell, it was a strange curve.