External pressure that is to be applied to the supercell

A pressure tensor is used to specify any external pressure that is to be applied to the cell

%BLOCK EXTERNAL_PRESSURE
units

$\begin{array}{ccc} \begin{displaymath} R_{xx} & R_{xy} & R_{xz}\\ & R_{yy} & R_{yz}\\ & & R_{zz}\\ \end{array}$

%ENDBLOCK EXTERNAL_PRESSURE

Here we have, for example $R_{xx}$ - the $xx$-component of the pressure and $R_{yz}$ the $yz$-component of the pressure and so on. The default units are GPa. If the whole BLOCK is omitted then no external pressure is applied by default.

For example we might specify

%BLOCK EXTERNAL_PRESSURE
atm
       1   0   0
           1   0
               1
%ENDBLOCK EXTERNAL_PRESSURE

Which will apply a hydrostatic pressure of 1 atmosphere upon the supercell from all three directions