I am not able to do a MOPAC minimization on some models.
There is a hard-coded heavy atom (non-Hydrogen) limit in CS MOPAC, due to the amount of memory required to run a MOPAC calculation over 60 heavy atoms. The amount of memory required for a CS MOPAC job each time you add a heavy atom to the model increases exponentially, not linearly. The heavy atom limit in the latest version of Chem3D Windows is 120 and 60 for Chem3D Mac. In Chem3D 4.0 Mac and Windows, the limit is 60.