When I try to run a MOPAC input file, I get the following error message: "Error from MOPAC: NO ATOMS IN SYSTEM"
If you created the Input file with something other than Chem3D, it could be that the formatting of the file is causing the problem. Open the file in MS Word, and save it as a Text file. If you are on Windows, make sure the file has the appropriate extension. Retest the problem.