Chem3D Users Manual - CambridgeSoft

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Administrator improvements in each method were designed to make global improvements, they have been found to be limited in certain situations. The two major questions to consider when choosing a potential function are: • Is the method parameterized for the elements in the model • Does the approximation have limitations which render it inappropriate for the model being studied For more detailed information see the MOPAC online manual. The following table shows the diatomic pairs that are parameterized in MINDO/3. An x indicates parameter availability for the pair indicated by the row and column. Parameters of dubious quality are indicated by (x). MINDO/3 Applicability and Limitations MINDO/3 (Modified Intermediate Neglect of Diatomic Overlap revision 3) is the oldest method. Using diatomic pairs, it is an INDO (Intermediate Neglect of Diatomic Orbitals) method, where the degree of approximation is more severe than the NDDO methods MNDO, PM3 and AM1. This method is generally regarded to be of historical interest only, although some sulfur compounds are still more accurately analyzed using this method. MNDO Applicability and Limitations 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 168•MOPAC Computations <strong>CambridgeSoft</strong> MOPAC Semi-empirical Methods
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/<strong>Chem3D</strong> MOPAC Computations • 169 MOPAC Semi-empirical Methods
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