Data items in the CHEMICAL_FORMULA category would not, in
               general, be used in a macromolecular CIF. See instead the
               ENTITY data items.

               Data items in the CHEMICAL_FORMULA category specify the
               composition and chemical properties of the compound. The formula
               data items must agree with those that specify the density,
               unit-cell and Z values.

               The following rules apply to the construction of the data items
               _chemical_formula.analytical, _chemical_formula.structural and
               _chemical_formula.sum. For the data item
               _chemical_formula.moiety, the formula construction is broken up
               into residues or moieties, i.e. groups of atoms that form a
               molecular unit or molecular ion. The rules given below apply
               within each moiety but different requirements apply to the way
               that moieties are connected (see _chemical_formula.moiety).

               (1) Only recognized element symbols may be used.

               (2) Each element symbol is followed by a 'count' number. A count
                  of '1' may be omitted.

               (3) A space or parenthesis must separate each cluster of (element
                  symbol + count).

               (4) Where a group of elements is enclosed in parentheses, the
                  multiplier for the group must follow the closing parenthesis.
                  That is, all element and group multipliers are assumed to be
                  printed as subscripted numbers. (An exception to this rule
                  exists for _chemical_formula.moiety formulae where pre- and
                  post-multipliers are permitted for molecular units.)

               (5) Unless the elements are ordered in a manner that corresponds
                  to their chemical structure, as in
                  _chemical_formula.structural, the order of the elements within
                  any group or moiety should be:  C, then H, then the other
                  elements in alphabetical order of their symbol. This is the
                  'Hill' system used by Chemical Abstracts. This ordering is
                  used in _chemical_formula.moiety and _chemical_formula.sum.