Constraints on the cell size/shape

The default is to allow all of the supercell lattice vectors to vary is size and orientation. To fix the size and shape of the supercell we must include the keyword

FIX_ALL_CELL : TRUE

There is also the possibility of allowing only some of the supercell lattice vectors/angles to vary while keeping others fixed

The BLOCK format is

%BLOCK CELL_CONSTRAINTS

$\begin{array}{ccc} \begin{displaymath} I_{a} & I_{b} & I_{c} \\ I_{\alpha} & I_{\beta} & I_{\gamma} \end{array}$

%ENDBLOCK CELL_CONSTRAINTS

The first line contains constraint information for the lattice vectors and the second line contains constraint information for the angles. First, if any entry is zero that quantity stays fixed. If two entries on a line are the same integer then they must both have the same value whatever that might be.