How to calculate a reaction's DeltaH using a MM forcefield calculator software?
What you are forgetting is that the reaction will also involve hydrogen.
$\ce{C6H6 + 3 H2 -&gt; C6H12}$
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.