<tt>extforce = [ <i>n   &nbsp; f<sub>x1</sub> f<sub>y1</sub> f<sub>z1</sub>
<br> &nbsp; &nbsp; &nbsp; &nbsp;  f<sub>x2</sub> f<sub>y2</sub> f<sub>z2</sub>
<br> &nbsp; &nbsp; &nbsp; &nbsp;  &hellip;
<br> &nbsp; &nbsp; &nbsp; &nbsp;  f<sub>xn</sub> f<sub>yn</sub> f<sub>zn</sub> </i> ] </tt>

<p>
<tt><i>n</i></tt> is the number of groups affected by this setting.
If <tt><i>n</i></tt> > 0, then atoms beloning to group <tt>k</tt> will experience
an additional external force <tt><i> f<sub>xk</sub> f<sub>yk</sub> f<sub>zk</sub> </i></tt>.
