This type specifies the Morse potential with 2-dimensional
periodic boundary conditions.
The free surfaces are perpendicular to the direction at the top and bottom
of the cell.
The range parameter, , the box lengths in the and
directions and the cutoff (as a fraction of the smallest box length)
must be specified using the *PARAMS* keyword in the `odata` file.
If the keyword *PRESSURE* is set in the `odata` file
then a constant pressure optimisation is performed
with the energy being minimised by eigenvector-following with respect to the
box lengths at every step.
The cutoff is automatically changed in proportion to the change in box size.
Otherwise the optimisations are performed at constant volume, with no
relaxation of the unit cell.
Subroutines used: **msdiff**, **mslatmin**.

David Wales
2018-06-23