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Hill notation

Frances Bernstein (bernstei@fcb.pdb.bnl.gov)
Wed, 25 Oct 1995 17:25:24 -0400 

    The mmCIF dictionary says, under _chem_comp.formula:

               4. The order of elements with the formula should be: C, then H,
                  then the other elements in alphabetical order of their
symbol.
                  This is the 'Hill' system used by Chemical Abstracts.

The PDB uses C, H, N, O, then others alphabetically.  Clearly it is simple
to convert one to the other but I got curious and looked at Chem Abstracts
for a description of the Hill system.  Apparently the ordering is as presented
in the mmCIF dictionary if carbon is present.  If there is no carbon, then
the ordering is strictly alphabetic; i.e., hydrogen is not given first.
Perhaps the description given in the dictionary should be revised to cover
the case where no carbon is present.

                                          Frances C. Bernstein