PM3 semi-empirical method

In large systems there can be many orbitals in a small energy range, and the size of the Cl matrix can be very sensitive to the value of the maximum excitation if you use Biergy Criterion. Since calculation time depends heavily on the size of the Cl matrix, you can end up with very long calculations, especially if you use the ab initio methods or the MNDO, AMI, or PM3 semi-empirical methods. This could exhaust the memory of your system. Again, inspecting the results of an RHF (no Cl) calculation will help you avoid these pitfalls. 

The researchers established that the potential energy surface is dependent on the basis set (the description of individual atomic orbitals). Using an ab initio method (6-3IG ), they found eight Cg stationary points for the conformational potential energy surface, including four minima. They also found four minima of Cg symmetry. Both the AMI and PM3 semi-empirical methods found three minima. Only one of these minima corresponded to the 6-3IG conformational potential energy surface. 

The basis set is 6-31G(d,p), and electron correlation at the MP2 level is included. A similar structure is obtained with the AMI and PM3 semi-empirical methods. Density functional theory at the B3LYP/6-31G(dp,p) level also produced the same structure for this ion-pair. The only observed differences between the semi-empiri-cal and the ab initio structures were slightly shorter hydrogen bonds (PM3 and AMI) between FI, F2, and F5 and the G2-F1 (H18) on the imidazolium ring. 

Ab initio and PM3 semi-empirical methods have been used to optimize the geometry of the benzo[l,5-r 2,4-r ]-dithiolane 4 (R = H or Et). Electron density calculations predicted desymmetrization in the radical cation <1997PCB3665, 1997SM(86)1999>. 

There is no doubt that a cycloadduct can be obtained in the self-initiation of styrene-maleic anhydride, although this is not necessarily a concerted reaction. A semi empirical calculus comparing the energy difference between reactants (styrene and maleic anhydride) and their cycloaduct and between two styrene molecules and their cycloadduct was carried out with the PM3 semi- empirical method provided by the Hyperchem program. Molecular geometries were calculated initially by molecular mechanics and afterwards by the PM3 calculation, at 0.01 convergence limits. The results were AE = 3. l Kcal/mol for the S-MA cycloadduct and AE = 122.8 Real/ mol for the Mayo styrene cycloadduct. However, the formation of the biradical of styrene requires 11.7 Kcal/mol and that of S-MA requires only 5.5 kcal/mol. The resonance stabilization of the biradical would easily produce the cycloadduct. 

The price for more general parameterization is decreased accuracy. Using the PM3 semi-empirical method, the mean absolute difference between calculated and experimental Ahyd// is 8.9 kJ mol" for the 10 randomly chosen cyclic and acyclic alkenes, alkadienes plus benzene used as a reference group for evaluating MM calculations above. The difference between calculated and experimental results in this group is larger with increasing unsaturation. 

Table 9-8. Bond distances calculated using AMI and PM3 semi-empirical methods. All bond distances are in A. Data obtained from Wavefunction, Inc. with permission.

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