When <tt>input = [ i j &alpha; ]</tt> is set before calling this command, then

<p>
change the supercell <b>H</b> while keeping the real
coordinates of the atoms <b>r</b><sub>k</sub> constant.  The scaled
coordinates <b>s</b><sub>k</sub> change because
<b>r</b><sub>k</sub> = <b>H</b> &sdot; <b>s</b><sub>k</sub>.

The matrix <b>H</b> consists of three column vectors
[<b>c</b><sub>1</sub> | <b>c</b><sub>2</sub> | <b>c</b><sub>3</sub> ], and will be changed by
<b>c</b><sub>i</sub> = <b>c</b><sub>i</sub> + &alpha; <b>c</b><sub>j</sub>.

<p>
Same as <tt>redefinepbc</tt>.   Also see <tt>changeH_keepS</tt>.
