.. _Z:

``Z``
=====

By default, the number of mers, Z, in a unit cell is worked out from the
empirical formula.  Thus, if a unit cell has the empirical formula
C\ :sub:`24`\ H\ :sub:`24`, then Z=24.  In those cases where the
empirical formula should not be used, e.g., benzene,
C\ :sub:`6`\ H\ :sub:`6`, the number of mers can be explicitly defined
by use of Z=n, here Z=4. Be careful to distinguish between the number of
mers in a unit cell and the number in a cluster - Z=n applies to the
number in the unit cell.

If Z=n should be used, but is not used, then the heat of formation per
formula unit will be in error by a factor, in the case of crystalline
benzene, the ΔH\ :sub:`f` per formula unit would be 1/6 what it should
be.