The formula for the reference entity. Formulae are written according to the rules: 1. Only recognised 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 element symbol and its count, but in general parentheses are not used. 4. The order of elements depends on whether or not carbon is present. If carbon is present, the order should be: C, then H, then the other elements in alphabetical order of their symbol. If carbon is not present, the elements are listed purely in alphabetic order of their symbol. This is the 'Hill' system used by Chemical Abstracts.