Chem3D Users Manual - CambridgeSoft

and other atoms are predicted. In particular,
simple H-bonds are generally not predicted to
exist using MNDO.
• Hypervalent compounds are too unstable, for
example, sulfuric acid.
• Activation barriers are generally too high.
• Non-classical structures are predicted to be
unstable relative to the classical structure, for
example, ethyl radical.
• Oxygenated substituents on aromatic rings are
out-of-plane, for example, nitrobenzene.
• The peroxide bond is systematically too short
by about 0.17 Å.
• The C-O-C angle in ethers is too large.
AM1 Applicability and Limitations
• In general, errors in ∆H f obtained using
AM1 are about 40% less than those given by
MNDO.
• AM1 phosphorus has a spurious and very
sharp potential barrier at 3.0Å. The effect of
this is to distort otherwise symmetric
geometries and to introduce spurious
activation barriers. A vivid example is given
by P 4 O 6 , in which the nominally equivalent
P-P bonds are predicted by AM1 to differ by
0.4Å. This is by far the most severe
limitation of AM1.
• Alkyl groups have a systematic error due to
the heat of formation of the CH 2 fragment
being too negative by about 2 kcal/mol.
• Nitro compounds, although considerably
improved, are still systematically too
positive in energy.
• The peroxide bond is still systematically too
short by about 0.17Å.
PM3 Applicability and Limitations
PM3 (Parameterized Model revision 3) may be
applied to the elements shaded in the following
table:
Important factors relevant to AM1 are:
• AM1 is similar to MNDO; however, there are
changes in the core-core repulsion terms and
reparameterization.
• AM1 is a distinct improvement over MNDO,
in that the overall accuracy is considerably
improved. Specific improvements are:
• The strength of the hydrogen bond in the
water dimer is 5.5 kcal/mol, in accordance
with experiment.
• Activation barriers for reaction are markedly
better than those of MNDO.
• Hypervalent phosphorus compounds are
considerably improved relative to MNDO.
The following apply to PM3:
• PM3 is a reparameterization of AM1.
• PM3 is a distinct improvement over AM1.
• Hypervalent compounds are predicted with
considerably improved accuracy.
ChemOffice 2005/Chem3D MOPAC Computations • 169
MOPAC Semi-empirical Methods