Evolution of molecular force fields

When the molecular mechanics method is applied in its full functional extension, it has moved a long way from the initial assumption of a minimization of vibrational strain energy, and there are a large number of parameters to be adjusted. Once one starts adjusting parameters, the method becomes parametric, and there is no point in defending first-principle derivations or in preserving a strict correspondence between molecular mechanics force constants and spectroscopic force constants. The most sensible attitude is to readjust all parameters all together, and this is what has been done, since the pioneer years, in more recent times for the preparation of the most popular and widely applied force fields. One is not restricted to functional forms that 

A special problem, of crucial importance in applications of molecular mechanics to biological problems, is the treatment of the hydrogen bond, an interaction which is halfway between a true chemical bond and a non-bonded interaction. This complication was tackled in an entirely practical attimde, using empirical formulas for a given A... H-D interaction A being the acceptor atom and D the donor, the CHARMM force field used the following  

All these force fields are embedded into computer packages and appropriate parameters are automatically called for when the user specifies the desired molecule. Many of the computer packages so far mentioned are available to academic users for a nominal fee, others appear in commercial packages that may sell for very high prices. 

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