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 ?


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

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

In principal, 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.


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