Complex phase behaviour from simple potentials

Simple pair potentials, where the term 'simple' denotes potentials that are isotropic and refer to one-component systems, can be used to describe effective interactions among substances with supramolecular architecture. By suitably choosing the functional dependence on the intermolecular distance, through such potentials it is possible to take into account, in an average way, the effect of the internal degrees of freedom of the macromolecules. This may give rise to phase phenomena that are radically different from those characterizing typical monoatomic systems. Here we review a number of simple model potentials presenting unusual, i.e. not argon-like, features and discuss the role of attraction and repulsion in determining their phase behaviour.