(1) Verify that MOPAC has already been installed and is working.
(2) Download the MOPAC App, and put it in the /Applications folder on the Mac.
(3) Double-click on /Applications/MOPAC.
The application will not open because it was not downloaded from the Apple store.
Instead, there will be an error indicating that security will not allow it to be opened.
If there is the option "Open", click on that.
Otherwise, go to System Preferences => Security & Privacy => click on "Open anyway" for MOPAC
(4) "Drag and drop" the MOPAC icon from the applications folder to the
This will produce an icon that has a small arrow on the desktop.
(5) Open a folder that contains a simple MOPAC data-set.
"Drag and drop" the data-set onto the MOPAC icon on the desktop.
An output file should appear in the folder, indicating that the job is running.
MOPAC can also be opened using the Mac "Open with" option.