See the CHEMICAL_FORMULA category description for the rules for
writing chemical formulae for inorganics, organometallics, metal
complexes etc., in which bonded groups are preserved as
discrete entities within parentheses, with post-multipliers as
required. The order of the elements should give as much
information as possible about the chemical structure.
Parentheses may be used and nested as required. This formula
should correspond to the structure as actually reported, i.e.
trace elements not included in atom-type and atom-site data
should not be included in this formula (see also