Slab boundary conditions (IMCON=6)

Slab boundaries are periodic in the X- and Y-directions, but not in the Z-direction. They are particularly useful for simulating surfaces. The periodic cell in the XY plane can be any parallelogram. The origin of the X,Y atomic coordinates lies on an axis perpendicular to the centre of the parallelogram. The origin of the Z coordinate is where the user specifies it, but at or near the surface is recommended.

If the XY parallelogram is defined by vectors $\mbox{$\underline{A}$}$ and $\mbox{$\underline{B}$}$, the vectors required in the CONFIG file are: (A$_{1}$,A$_{2}$,0), (B$_{1}$,B$_{2}$,0), (0,0,D), where D is any real number (including zero). If D is nonzero, it will be used by DL_POLY to help determine a `working volume' for the system. This is needed to help calculate RDFs etc. (The working value of D is in fact taken as one of: 3$\times$cutoff; or 2$\times$max abs(Z coordinate)+cutoff; or the user specified D, whichever is the larger.)

Note that the standard Ewald sum cannot be used with this boundary condition. DL_POLY_2 switches automatically to the Hautman-Klein-Ewald method instead [32].

The surface in a system with charges can also be modelled with DL_POLY_2 if periodicity is allowed in the Z-direction. In this case slabs of ions well-separated by vacuum zones in the Z-direction can be handled with IMCON=2 or 3.