When does each type of MOPAC calculation apply, and how accurate are they?
In general, ChemBio3D's online help is intended to help use MOPAC, rather than to address technical details like this question. Hence, you'll find references in our online help to the full MOPAC documentation manual at http://www.openmopac.net/
MOPAC's full documentation includes in its "Geometry" section a definition of applicable elements for each Hamiltonian. For example, the AM1 method does not apply to transition metals, while the PM6 method does.
On MOPAC's accuracy: MOPAC's full documentation included in its "Accuracy" section a listing of average errors for numerous properties, from bond angles to heat capacity and entropy average errors.
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