Strengths and Weaknesses of Ab initio Calculations

Compared to other methods (molecular mechanics, semiempirical calculations, density 

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List the strengths and weaknesses of ab initio calculations compared to molecular mechanics and extended Hiickel calculations. State the molecular features that can be calculated by each method. 

The performance is (as expected) very good. MMX provides relative (and absolute) stabilities with a MAD of only 1.2 kcal/mol, which is better than the estimates from the combined theoretical methods in Table 11.31. Considering that force field calculations require a factor of 10 less computer time for these systems than the ab initio methods combined in Table 11.31, this clearly shows that knowledge of the strengths and weakness of different theoretical tools is important in selecting a proper model for answering a given question. 

As the name implies, semi-empirical calculations simplify the computational problem presented by the Schrodinger equation. Semi-empirical approaches eliminate some of the direct number crunching involved in ab initio calculations and instead replace portions of the calculation with values that may be taken from experimental data or other calculations that are parameterized to agree with empirical data. There is a variety of semi-empirical schemes that differ in the types of parameterizations that are made. Three common semi-empirical methods that are included in Spartan02 are called AMI, PM3, and MNDO. Each has strengths and weaknesses depending on the specific molecular environment one wishes to model. 

The calculated and experimental results, shown in Table 1, closely follow the same trend. The cooperative effects are demonstrated by the N - H stretching frequencies. Relative to Id, ab initio calculation indicates that the formation of a two-center H-bond in a five-membered ring in lc produces a small blueshift of the N - H stretching frequency, indicating the weakness of this two-center H-bond which alone does not have the strength to produce the expected redshift. IR experiment showed that the N - H stretching fre-... 

A semi-empirical method which takes a somewhat different track is the MINDO method of Dewar (Bingham and Dewar, 1972). This method differs from previously mentioned semi-empirical methods in that it seeks to reproduce not the ab initio results which are in some respects inaccurate anyway, but rather the experimental results. The formalism is similar to that used in a CNDO calculation. However, by adjusting other quantities in the calculation, an effort is made to compensate for the lack of explicit inclusion of electron correlation. There are both strengths and weaknesses to this method. When suitably parameterized, it may give results which arc more useful and dependable than those obtained by ab initio calculation. At present this can be done very well in some respects, not so well in other respects. The computation time is similar to that required by the CNDO method. 

It is possible to use computational techniques to gain insight into the vibrational motion of molecules. There are a number of computational methods available that have varying degrees of accuracy. These methods can be powerful tools if the user is aware of their strengths and weaknesses. The user is advised to use ab initio or DFT calculations with an appropriate scale factor if at all possible. Anharmonic corrections should be considered only if very-high-accuracy results are necessary. Semiempirical and molecular mechanics methods should be tried cautiously when the molecular system prevents using the other methods mentioned. 

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