To avoid ``wall effects''we surround the basic cell containing
the particles by periodic images of itself.

Spin lattices:
In each coordinate direction the last (``rightmost'') spin in a row
interacts also with a right neighbor, which is taken to be identical to
the first (leftmost) spin in that row, and vice versa (see Figure).

Figure 2.1:
Periodic boundary conditions on a spin lattice

Fluid: Apply the following rule:

For each particle store, instead of any coordinate ,
the quantity

(with the side length of the cell).
When a particle leaves the cell to the right,
it is automatically replaced by a particle entering from the left, etc.

Nearest Image Convention:
In computing pair vectors between two particles
and one needs only differences of coordinate values.
If this coordinate difference
is larger than , then
the particle will not be regarded as an interaction partner of ;
instead, its left periodic image with coordinate interacts
with .

The NIC rule may be implemented by a sequence of IF commands or
by the more compact code line

where nint() denotes the rounded value of , i.e. the integer nearest
to . This formulation of NIC is also better suited for vectorising
compilers.
F. J. Vesely / University of Vienna