Existing Force Fields

These minimum number of variables that determine the state of a system are called the independent variables, and all other variables which can be functions of the independent variables are dependent variables or thermodynamic functions. For a system where no external force fields exists such as an electric field, a magnetic field and a gravitational field, we normally choose as independent variables the combination of pressure-temperature-composition or volume-temperature-composition. 

Some quantity which will act as a measure of the magnitude of the body force acting on the flow. Consideration here will only be given to buoyancy forces, i.e., to forces arising dueio gravity. However, the method used here to derive a measure of this force can easily be applied when other force fields exist. 

Many of these force fields exist in several versions, and one should specify the version used in any calculation. 

Molecules are described by a ball and spring model in force field methods, with atoms having different sizes and softness and bonds having different lengths and stiffness . Force field methods are also referred to as molecular mechanics (MM) methods. Many different force fields exist, and in this Chapter we will use Allinger s MM2 and MM3 (Molecular Mechanics versions 2 and 3) to illustrate specific details. ... 

The kind of energy terms, their functional form, and how carefully (number, quality, and kind of reference data) the parameters were derived determine the quality of a force field. Accurate force fields exist for organic molecules (e.g., MM2, MM3), but more approximate force fields (e.g., with fixed bond distances) optimized for computational speed rather than accuracy [e.g., AMBER (assisted model building with energy refinement), CHARMM (chemistry at Harvard molecular mechanics), GROMOS (Groningen molecular simulation)] are the only practical choice for the treatment of large biomolecules. The type of molecular system to be smdied determines the choice of the force field. 

Only if the integral / / eid dr, has the same value for all paths between the initial and final positions of the particle does a conservative force field exist otherwise the concept of a potential energy is not valid. 

A great variety of force fields exist, with respect to functional form, to type, and calibration of parameters and to the data used for calibration. The GROMOS force field is one of the simplest forms of a force field for molecular systems. It can be refined and extended in various ways, but a more complex form does not necessarily imply a better force field. If the standard force field does not contain parameters for particular types of atoms, the user should choo.se parameters that are as consistent as possible with the standard force field. The following three common sources of inaccuracy when choosing additional force field parameters are worth mentioning... 

---