An integer array specifying whether atom <tt>i</tt> is fixed (<tt>fixed[i] = 1</tt>)
or is free (<tt>fixed[i] = 0</tt>).  The atom is considered to be removed (i.e. not
included in the calculation of potential energy and forces) if <tt>fixed[i] = -1</tt>.

<p>
Also see <tt>species</tt>, <tt>group</tt>, <tt>fixatoms_by_ID</tt>, <tt>fixatoms_by_position</tt>.
