Use of force fields derived from classical mechanics

6 Use of force fields derived from classical mechanics 

Many systems of interest are too large to be tackled using ab initio methods and here force field methods can be useful. Force field methods do not explicitly include the electrons, rather the energy of a system is a function only of the nuclear coordinates. The main application of molecular mechanics modelling is in the area of big systems (thousands of atoms are not uncommon). The calculations can be performed in a fraction of the computer time that would be required for an ab initio calculation. Their accuracy is determined by the quality of the parameterization of the force field. 

The model of the interactions in molecular mechanics is simple and the Bom-Oppenheimer approximation must be applicable. The most common molecular modelling force fields are based on the very simple picture of intra- and intermolecular forces. The energy is expressed as a function of a series of reference values for the bond lengths and angles. The electrostatic interactions can also be considered in this framework. 

The most common force fields include two-, three- and four-body potential energy functions. It is possible to use more complicated force 

The force fields have to be defined through their functional form as well as the parameters. Different force fields can use the same functionality with very different parameters (or force fields with different functional forms) and still provide results of comparable accuracy. This highlights the problem of transferability of force fields and parameters. Force fields are often developed to study certain characteristics and properties of materials and that information forms part of the force field itself. Force fields that are derived to reproduce the structural properties of solids and molecules do not always provide good results when calculating vibrational spectra. This does not mean that the force field fails it is simply being used outside its range of applicability. 

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