2.2. Molecular formula search

        Molecular formula search allows one to find molecules that contain a certain number and type of elements.  Typically, molecular formula search returns by default molecules that exactly match the queried stoichiometry.  For example,   a query of “C₆H₆” will return all structures containing six carbon atoms, six hydrogen atoms and nothing else.  However, molecular formula search implemented in some databases, including PubChem Chemical Structure Search, has an option to allow other elements in returned hits (e.g., C₆H₆O or C₆H₆N₂O for the “C₆H₆” query).