What data do you have? Do you know the mechanisms?
We calculated heats of formation in our chemical thermodynamics course, but we were always given ample data, and we knew what we were trying to calculate.
We calculated heats of formation in our chemical thermodynamics course, but we were always given ample data, and we knew what we were trying to calculate.
If you think you can do the same thing for ethene -> butadiene, you'll find yourself to be horribly wrong, because of extended conjugation networks.
So while for some cases it works, it is not always simple to go from known empiricial results to more complex structures using tables and addition and subraction. And in the case of the molecules I care about, there is pretty good reason to believe that the simple linear methods will fail.