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• 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
MNDO Applicability and Limitations
Important factors relevant to AM1 are:
The following limitations apply to MNDO:
• Sterically crowded molecules are too unstable,
for example, neopentane.
• Four-membered rings are too stable, for
example, cubane.
• Hydrogen bonds are virtually non-existent, for
example, water dimer. Overly repulsive
nonbonding interactions between hydrogens
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.
• 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.
• 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
ChemOffice 2005/Chem3D Computation Concepts • 145
Approximate Hamiltonians in MOPAC