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This type specifies the Morse potential with 2-dimensional periodic boundary conditions. The free surfaces are perpendicular to the z direction at the top and bottom of the cell. The range parameter, ρ0, the box lengths in the x and y 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.