Introduction

The range of problems amenable to study using MD knows few bounds and as computers become more powerful the range continues to expand. Because of the enormous breadth of the subject, we have chosen to concentrate on the simplest of systems and avoid overly specialized models. Most of the case studies up to this point have been based on short-ranged, two-body interactions; within this framework a considerable variety of problems can be studied, but a few conspicuous gaps remain. Pair potentials have their limitations, and while certain kinds of intermolecular interaction can be imitated by the appropriate combinations of pair potentials, it is sometimes essential to introduce many-body interactions to capture specific features of the ‘real’ intermolecular force [mai81].

In this chapter we present two different approaches to the introduction of many-body interactions, namely, three-body interactions and the embedded-atom method, each in the form of a case study. We cannot do justice to the range of applications to which these and other enhancements of the MD method, such as those discussed in Chapter 13, have contributed, but in the prevailing culinary atmosphere we hope the reader will gain at least a taste of what is involved.

Three-body forces

The problem

Even when regarded simply as effective potentials, the capacity of the pair potential to reproduce known behavior has its limitations.