Consider the example in Figure , here, either hydrogen atom H 1 or H 2 is transferd to the hydroxy group. From a chemical point of view, hydrogen atoms H 1 and H 2 are equivalent, even though this equivalence is not readily apparent to computational methods tasked with inserting hydrogens. Such methods may yield two alternative reaction centers depicted in Figure A and B. In this case, these graphs are isomorphic, indicating that the reaction centers in A and B are interchangeable. The reaction in Figure shows, however, that there is not always a unique hydrogencompletion of the ITS. In this particular example, the resulting reaction center graphs, Figure and Figure , are not isomorphic, highlighting the complexity and ambiguity inherent in achieving hydrogen completeness.