E.g. what compound is more stable SmF3 or Sm2O3? Based on e.g. HSC software calculation one can get delta H (of formation) for both compounds and the lower value (more negative) indicates the more stable compound. BUT in SmF3 we have four atoms and in Sm2O3 five atoms. Why this seems to be a problem? When we want to compare the stability of several MF2 fluorides (e.g. M=Fe, Ni, Co, Zn, ...) with NF3 fluorides (e.g. N=Al, Y, La...Lu) probably all NF3 compounds will have lower delta H values and should be more stable. And this is the problem, not all NF3 compound are more stable then MF2 compounds.

What is the correct procedure how to compare the stability of different compounds?