Relative Energies of C4H6 Isomers

MINDO/3 methods clearly have severe problems with some of the conjugated systems. The MNDO/AM1/PM3 family perform somewhat better, although none of them can predict the correct ordering. The SAMI method is not an improvement for this case. The mean absolute deviation (MAD) for the predicted stabilities is 10kcal/mol, which is a typical accuracy for semi-empirical methods. 

The next step up in terms of theory is ab initio FIF with increasingly larger basis sets. Table 11.26 shows the results for various basis sets, where the geometries have been optimized with the STO-3G, 3-2IG and 6-31G(d,p) basis sets, but with the latter used for the larger basis. The minimum STO-3G basis performs worse than the semi- 

In order to improve the results further, several features need to be addressed 

These effects are shown in Tables 11.27-11.30. The energetic changes due to optimization at the MP2 level, relative to the HF geometries, are less than 0.7 kcal/mol (Table 11.27). 

The influence of zero-point energies is shown in Table 11.28. As the HF frequencies are systematically too high, the HF/6-31G(d,p) values are scaled by 0.92, and the MP2/ 6-31G(d,p) values are similarly scaled by 0.97. The change in stabilities by zero-point energy corrections is less than 2 kcal/mol, and the difference between the HF and MP2 values is less than 0.7 kcal/mol. 

There are experimental values for the first eight structures, which allows an evaluation of the performance of different methods. This in turn enables an estimate of how much trust should be put in the predicted values for structures 9,10 and 11. 

Addition of electron correlation beyond MP2 improves the agreement with experiments to 4kJ/mol, as shown in Table 11.26, and essentially all of the advanced methods provide similar accuracy for these (uncompUcated) systems. The composite G3 method that tries to estimate the QCISD(T)/6-311-H-G(2df,2p) results by additivity of lower level calculations provides similar results. From the observed accuracy of 4kJ/mol for structures 2-8, the energetics of species 9-11 may be assumed to be reliable to the same level of accuracy. 

If further refinements are required, several factors must be considered  

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Table 11.23 Harmonic frequencies for O3 with the 11.8 Relative Energies of C4H6 Isomers ...

The Vibrational Frequencies of O3 Relative Energies of C4H6 Isomers References... 

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