Compute Hessian matrix by numerical differentiation.

<p>
The step size in (&Aring;) for numerical differentiation
is specified by <tt>timestep</tt>.

Result saved in <tt>hessian.out</tt> in the current directory. 
Note that the results are actually the negative of the Hessian matrix,
i.e. it is the first derivative of the forces that are computed.

<p>
To compute the free energy based on Harmonic Approximation, use
<tt>MD++/Tools/matlab/free_eng_ha.m</tt> as a starting point.

<p>
If <tt>input[0]=0</tt>, then the entire Hessian matrix is computed.

<p>
If <tt>input[0] &ne; 0</tt>, then <tt>input[0] = n</tt> specifies the number
of atoms whose corresponding (3) columns in the Hessian matrix will
be computed.  The atom indices are specified in <tt>input[1]</tt>,
&hellip; , <tt>input[n]</tt>.

<p>
Also see: Computer Simulations of Dislocations
<a href="http://micro.stanford.edu/~caiwei/CSD-Book">web site</a>
Chapter 6, Section 6.2.
