Periodic ForceField
*******************

The forcefield engine can be used to optimize the geometries of
periodic systems. When specifying bonds, via the System%Bondorders key
block, one should also specify the bonds that pass through a cell
boundary. The GUI does this automatically. You can also simply set the
key System%GuessBonds to true, and then UFF-guessed bonds will be
used.

When you are having charges, Ewald summation will be used to calculate
the Coulomb interaction. Currently this will be fairly slow for 1D and
2D periodic systems, as the classical Ewald trick cannot be applied.
