A string instructing MD++ what variables to write to the property file.  Any variable
recognized by MD++ can included, e.g.<br> &nbsp;
 <tt>output_fmt = "curstep EPOT KATOM Tinst" </tt>

<p>
Example: <tt>MD++/scripts/Examples/example05-au.tcl</tt>

<p>
Also see <tt>saveprop</tt>, <tt>outpropfile</tt>.
