How to calculate a reaction's DeltaH using a MM forcefield calculator software?

Question

I want to calculate $\Delta H$ for hydrogenation of benzene and producing cyclohexane using MM+ Force Field Energy. I am using Hyperchem for energy calculation and optimization. I calculated the energy for cyclohexane and then I calculated the energy for benzene. But the difference of two energies is totally false. The calculated energy is $8~\mathrm{kcal/mol}$ however the real value is $-49~\mathrm{kcal/mol}$.

Is it a true way to calculate $\Delta H$? If not, how can I do it ?

Answer 1

What you are forgetting is that the reaction will also involve hydrogen.

$\ce{C6H6 + 3 H2 -> C6H12}$

In principle, the $\Delta H$ for $\ce{H2}$ should be 0, but in practice, that may not be true with a force field. The energies from molecular mechanics are seldom accurate enough to compute reaction energies. They are parameterized on a set of molecules, but the errors are usually large, both systematic and random errors.