An infinite system is represented by cell supplied by the user, called the
Central Unit Cell, or the CUC, and the cells surrounding this CUC. When
`DCART`, `LARGE`, and `DEBUG` are used in an infinite system
calculation for which the geometry has been optimized, the Cartesian
derivatives for all unit cells are output. Many of these will be quite large,
up to about 60 kcal/mol/Å. This is not an error, rather it is a peculiarity
of the way solid-state derivatives are stored.

The Cartesian derivatives of the CUC represent the sum of all forces acting on
the atoms of the CUC due to all the atoms in the CUC. Thus, if the atoms in
the CUC are the set (*a*,*b*,*c*,*d*,*e*,*f*), then the Cartesian derivatives
for atom *a* represent the forces on *a* due to the set
(*b*,*c*,*d*,*e*,*f*). The Cartesian derivatives of atom *a* do *NOT*
include terms from the surrounding unit cells. Because of this, those atoms
in the CUC which are at the cell boundaries are likely to have large
derivatives.

The Cartesian derivatives of the surrounding unit cells represent the forces acting on the atoms in those cells arising from the atoms of the CUC. Again, this is an unbalanced set of forces, and those atoms near to the cell boundaries are likely to have large resultant forces.

It is possible to evaluate the total, balanced, forces acting on the atoms of the CUC. This is done by simply adding the forces acting on the atoms of the three unit cells. When the keywords given above are used, the last part of the derivative output consists of the forces acting on the CUC itself.

Only by representing the forces in this unusual manner can the information necessary for calculating the derivative of the translation vector be generated.