Differences in Force Fields

There are many different force fields in use. They differ in three main aspects:

(1) What is the functional form of each energy term?

(2) How many cross terms are included?

(3) What types of information are used for fitting the parameters?

There are two general trends. If the force field is designed primarily to treat large systems, such as proteins or DNA, the functional forms are kept as simple as possible. This means that only harmonic functions are used for Estr and Ebend (or these term are omitted, forcing all bond lengths and angles to be constant), no cross terms are included, and the Lennard-Jones potential is used for Evdw. Such force fields are often called "harmonic", "diagonal" or "Class I". The other branch concentrates on reproducing small- to medium-size molecules to a high degree of accuracy. These force fields will include a number of cross terms, use at least cubic or quartic expansions of Estr and Ebend, and possibly an exponential-type potential for Evdw. The current efforts in developing small-molecule force fields go in the direction of not only striving to

Table 2.4 Comparison of functional forms used in common force fields;49 the torsional energy, £tors, is in all cases given as a Fourier series in the torsional angle

Force field

Types

Estr

Ebend

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