Semi-empirical Results

We had hoped that the semi-empirical calculations would maintain the conformer energies in the order predicted by MM2. Unfortunately this was not the case, although the energy differences were often smaller than those computed by MM2, especially for the smaller cage structures. In addition, the structure predicted to be the lowest-energy conformer by MM2 did not always maintain that status after semi-empirical optimization. Furthermore, the lowest energy structures computed by the three semi-empirical models did not always come from the same MM2 starting structure. In short, there was very little coherence between the MM2 and semi-empirical results. 

Clearly the semi-empirical COSNAR resonance energies are quite different from the corresponding MM2 energies. None of the semi-empirical methods predict more than - 10 kcal/mol of stabilization for any structure, and most of the resonance energies are 5 kcal/mol. Also, only the MNDO method matches the MM2 prediction that the resonance energy will be 

Resonance Energies from AMI, MNDO and PN3 Calculations with Energies for individual isodesmic model compounds (energies reported in kcal/mol for lowest energy conformers only)  

Svstem Method Amide Alkane Ketone Amine RE 

3 (28) BBLs have the least distorted linkages. However, only AMI predicts less total distortion than MM2. 

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The Wd estimated from the RASCI is compared with other theoretical calculations [46, 54] in Table V. The present DF estimate of Wd deviates by 21%(29%) from the SCF (RASSCF) estimate of Kozlov et al. [46] and by 17% from the semiempirical result of Kozlov and Labzovsky [54], while our RASCI result departs by 6% (3%) from the SCF-EO (RASSCF-EO) treatment of Kozlov et al. [46] and is in good agreement with the semi-empirical result of Kozlov and Labzovsky. At this juncture, we emphasize that our computed ground state dipole moment of BaF (p(. = 3.203 debyes) is also reasonably close to experiment p(. = 3.2 debyes (see Table 5 of Ref. 38). 

Equation (48) represents a rough estimate of the shift due to solid-state effects A d by subtracting the work function from the total shift. Our results for the atom-solid shift All agree quite well with the theoretical and semi-empirical results of Johansson and M rtensson83. ... 

These discrepancies may be solved by our test calculations using Pople s G 90 program system In agreement with optimizations of formamidine (cf 137) the minimal STO-3G basis set leads, as in the semi-empirical result of Figure 2, to a pyramidal configuration of the type 117 at nitrogen as an energetically more stable form, if... 

Breindl et. al. published a model based on semi-empirical quantum mechanical descriptors and back-propagation neural networks [14]. The training data set consisted of 1085 compounds, and 36 descriptors were derived from AMI and PM3 calculations describing electronic and spatial effects. The best results with a standard deviation of 0.41 were obtained with the AMl-based descriptors and a net architecture 16-25-1, corresponding to 451 adjustable parameters and a ratio of 2.17 to the number of input data. For a test data set a standard deviation of 0.53 was reported, which is quite close to the training model. 

I hc choice of the O0 method depends on scvcrni faclors including your previous experience and preferences. If you want Lo compare the results lo other studies, you must use ihe same semi-empirical melhod. Since some tn elli ods can converge much more quickly than olh ers, you m igh l wan t to use a fast meth od lo obtain an approximation of the final answer, and then a more accurate method for the final lesulL,... 

For small molecules, the accuracy of solutions to the Schrtidinger equation competes with the accuracy of experimental results. However, these accurate a i initw calculations require enormous com putation an d are on ly suitable for the molecular system s with small or medium size. Ah initio calculations for very large molecules are beyond the realm of current computers, so HyperChern also supports sern i-em p irical quantum meclian ics m eth ods. Sem i-em pirical approximate solutions are appropriate and allow extensive cliem ical exploration, Th e in accuracy of the approxirn ation s made in semi-empirical methods is offset to a degree by recourse to experimental data in defining the parameters of the method. 

Semi-empirical methods, such as those outlined in Appendix F, use experimental data or the results of ab initio calculations to determine some of the matrix elements or... 

Nearly every technical difficulty known is routinely encountered in transition metal calculations. Calculations on open-shell compounds encounter problems due to spin contamination and experience more problems with SCF convergence. For the heavier transition metals, relativistic effects are significant. Many transition metals compounds require correlation even to obtain results that are qualitatively correct. Compounds with low-lying excited states are difficult to converge and require additional work to ensure that the desired states are being computed. Metals also present additional problems in parameterizing semi-empirical and molecular mechanics methods. 

HyperChem can plot orbital wave functions resulting from semi-empirical and ab initio quantum mechanical calculations. It is interesting to view both the nodal properties and the relative sizes of the wave functions. Orbital wave functions can provide chemical insights. 

You can also plot the electrostatic potential, the total charge density, or the total spin density determined during a semi-empirical or ab initio calculation. This information is useful in determining reactivity and correlating calculational results with experimental data. These examples illustrate uses of these plots ... 

The accuracy of a molecular mechanics or semi-empirical quantum mechanics method depends on the database used to parameterize the method. This is true for the type of molecules and the physical and chemical data in the database. Frequently, these methods give the best results for a limited class of molecules or phenomena. A disadvantage of these methods is that you must have parameters available before running a calculation. Developing parameters is time-consuming. 

Semi-empirical quantum mechanics methods have evolved over the last three decades. Using today s microcomputers, they can produce meaningful, often quantitative, results for large molecular systems. The roots of the methods lie in the theory of % electrons, now largely superseded by all-valence electron theories. 

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. 

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