%%%%%%%%%%%%%%%  -*-Tex-*-  %%%%%%%%%%%%%%%%%%
% short description of MD++ command
%  changeH_keepR

Requires {\tt input = [ i j $\alpha$ ]} to be set beforehand.
%
Change the supercell $\textbf{H}$ while keeping the real
coordinates of the atoms $\textbf{r}^{i}$ constant.  The scaled
coordinates $\textbf{s}^{i}$ change because
$\textbf{r}^{i}=\textbf{H}\cdot\textbf{s}^{i}$.
%
The matrix $\textbf{H}$ consists of three column vectors
$[\textbf{c}_1|\textbf{c}_2|\textbf{c}_3]$, and will be changed by
$\textbf{c}_i = \textbf{c}_i + \alpha\, \textbf{c}_j$.
%
Same as {\tt redefinepbc}.
