I get the following error message during a MOPAC calculation:
"Atom positioned using three co-linear atoms"
The error message indicates that you have a linear group of atoms in your molecule (like acetylene, for example). The problem is the 180 degree dihedral angle. You need to use the xyz keyword when running the MOPAC calculation on this molecule. For more information on the xyz keyword, please see section 3.3 of the MOPAC manual (manual can be found on your cd-rom in the Manuals folder).