There are a few places in Cantera where a model handles a particular species in a special way, e.g. H₂O and CO₂ in the flame radiatiaton model, and O₂^- in IonGasTransport and we currently do this based on matching the species name. However, there is generally no requirement that species be named in this way, so it would be useful if we could find these species by their atomic composition instead.

This could be done in C++ by adding an overload to Phase::speciesIndex which takes a compositionMap as the argument. For the Python interface, it could involve another elif check for a dict containing the composition.