Usage: <tt>input = [ <i>ng  group_1 group_2 &hellip; group_ng</i> ]  fixatoms_by_group<tt>

<p>
If <tt><i>ng</i></tt> > 0, then all atoms <tt>i</tt> whose <tt>group[i]</tt> value matches any of 
the <tt><i>group_1 group_2 &hellip; group_ng</i> </tt> will have <tt>fixed[i] = 1</tt> set.

<p>
Also see <tt>setfixedatomsgroup</tt>.
