If <tt>mcatom</tt> >= 0, then at each step of the Monte Carlo simulation, an atoms will be selected 
at random to be displaced by a small random vector (trial move).  <tt>mcatom</tt> then stores the
index of this selected atoms.

<p>
If <tt>mcatom</tt> = -1, then all atoms will be displaced by random vectors in the trial move.  This
generally leads to very low acceptance ratio unless the simulation cell is very small.
