Source-code patches that add the group's PDDG/PM3 and PDDG/MNDO semiempirical methods to MOPAC6 — a substantial accuracy gain at the same speed, with all of MOPAC's original methods retained.
Working out a molecule’s energy and shape from quantum mechanics is expensive. Semiempirical methods are a fast shortcut: they approximate the full calculation using parameters fit to experimental data, running orders of magnitude quicker. MOPAC is the classic program for them, with methods like PM3 and MNDO.
PDDG/PM3 and PDDG/MNDO are the Jorgensen group’s improved versions. A small “pairwise distance directed Gaussian” (PDDG) correction to the atom-core interactions makes them markedly more accurate — especially for heats of formation and geometries of organic molecules — while staying just as fast.
These are the FORTRAN 77 source patches that add PDDG/PM3 and PDDG/MNDO to MOPAC6 as new methods, so all of MOPAC’s original functionality is kept.
Get the MOPAC6 FORTRAN source (freely available from the CCL archive), replace the five files above with these, and recompile:
# put the 5 files in the MOPAC6 source dir, then:
makeIn your MOPAC input, use the new Hamiltonian keywords in place of PM3 / MNDO:
PDG # → PDDG/PM3
MDG # → PDDG/MNDONote: the patches add PDDG/PM3 and PDDG/MNDO as separate methods, so MOPAC6’s original AM1 / PM3 / MNDO functionality is fully retained. Written for unix/linux builds of MOPAC6.
Citation count from Google Scholar, August 2026. Map from the peer-reviewed research citing PDDG (Repasky, Chandrasekhar & Jorgensen, J. Comput. Chem. 2002; via OpenAlex). Map: simple-world-map, CC BY-SA 3.0.
Repasky, M. P.; Chandrasekhar, J.; Jorgensen, W. L. PDDG/PM3 and PDDG/MNDO: Improved Semiempirical Methods. J. Comput. Chem. 2002, 23, 1601–1622. doi:10.1002/jcc.10162