Hi, I’m using MMFF94 to calculate the energy of a single water molecule. It gives me a value that is effectively 0 kj/mol. I observe a similar calculation result when I do this for a single CO2 molecule. From a previous thread, I understand this should give me the heat of formation and because it is a classical force field, the results won’t be very accurate.
Out of curiosity, I tried methane and got something “significantly larger”: 0.11kj/mol. What’s the deal with all these small molecules giving such tiny heats of formation?
Let me specify, I’m using avogadro 1.2
Correct.
In general, these force fields have been parameterized on a set of molecules, generally much larger than what you’re considering. For example MMFF94 was developed for drug-like molecules.
As you noted, the results have a range of accuracy. Certainly molecules / geometries with lower force field energies tend to be more stable.
But it’s nowhere near the accuracy of any quantum calculation.
In principal, it should be the same no matter the version you’re using, but thanks for specifying.