The convenience method allows one to do this:

cclib/data/Gaussian/basicGaussian09$ python
>>> import cclib
>>> data = cclib.parser.ccopen('water_mp4.log').parse()
>>> data.stoichiometry()
'O1H2'

That's cool! One thing though: do we want to be explicit when there's only one atom of a given element? I find the "1" after the O really weird...

Here you go. Also made it compliant with the Hill system, which I think is the standard.

Having it be a property makes it work more like an attribute:

cclib/data/Gaussian/basicGaussian09$ python
>>> import cclib
>>> data = cclib.parser.ccopen('water_mp4.log').parse()
>>> data.stoichiometry
'O1H2'

LGTM!