2.3. Identity search

        Identity search is to locate a particular chemical structure that is “identical” to the query chemical structure.  Although identity search seems conceptually straightforward, one should keep in mind that the word “identical” can have different notions.  For example, if a molecule exists as multiple tautomeric forms in equilibrium, do you want to consider all these tautomers identical and search the database for all of them?  If your query molecule has a chiral stereo center, should you consider both R- and S-forms in your search?  In your identity search, do you want to include isotopically substituted species of the provided query molecule as well as the query itself?  Depending on how to deal with these nuances of chemical structures, identical search will return different results.  The identity search in the PubChem Chemical Structure Search allows users to choose a desired degree of “sameness” from several predefined options.  To see these options, one need to expand the options section by clicking the “plus” button next to the “option” section heading.