How do I open a MOPAC output file in Chem3D?
Open the .arc file in a text editor. Remove all the text above the keywords section of the .arc file, so the first line of the file now looks something like:
EF GNORM=0.100 MMOK GEO-OK PM3 T=1D
and then save the file with a .mop extension. Chem3D can open MOPAC Input .mop files.