Extension of The Alkane Force Field

The MM4 frequencies are adequate for the calculation of a variety of properties, for example, entropies (Chapter 11). It appears that much of the calculational error in the vibrational frequencies stems from the couplings between frequencies. Thus, one frequency is calculated too low and another too high. But, if one is interested in a molecular property that uses a summation over the frequencies (e.g., entropy), most of the error cancels out. 

Our conclusion is that the MM4 force field for alkanes can indeed be improved somewhat, but the improvements would be small, and there appears to be no strong reason for making such improvements at this time. Force fields for organic molecules beyond alkanes are another matter, and will be discussed briefly in the following pages. 

The alkanes constitute the most simple and fundamental class of organic molecules. They contain only two kinds of atoms, and only single bonds between those atoms. There are probably thousands, perhaps tens of thousands, of alkanes that are already known in the sense that they have been isolated and put into bottles. There are countless more that will be isolated later or that can be or have been investigated by computational 

There is an important point to be mentioned here. Saturated hydrocarbons (alkanes) are treated in MM4 as very classical systems, and, in particular, they do not contain charge separation or dipole moments. This is not exactly correct (discussed in the following) but seems to be a good enough approximation for everything that we know to date. But when we introduce functional groups into the molecule, dipole moments, and their resulting induction of polarization into molecules, will become important. Even alkenes, which have relatively small dipole moments (mostly less than 1D), require 

Another level of complexity beyond alkenes as we further extend molecular mechanics calculations would be that of conjugated hydrocarbons. These are indeed a special case and something quite beyond what has been covered up to this point. Conjugated hydrocarbons will be covered in the next chapter. 

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