I'm running MacOS 8.5.1 (or greater). When I try to minimize a structure using MM2, Chem3D 4.0 crashes.
Chem3D 4 was developed and released before MacOS 8.5.1 and above. Apparently, there is a conflict with the QuickTime 3.0.2 extension that comes with MacOS 8.5.1 and above. The solution is to upgrade to the latest version of Chem3D. The only other alternative is to use your Extensions Manager to turn off this extension. After disabling the QuickTime 3.0.2 extension and restarting your Mac, you will be able to use the MM2 minimization command.