Knowledge of the intermolecular potential for simple (i.e. monatomic) molecules has increased greatly in recent years. For polyatomic molecules, on the other hand, such knowledge is still rather meagre, and much more is needed. One needs to know (i) what is the pair potential? (ii) how important are the triplet and other multibody potentials in liquids? These multibody potentials have been studied very little for polyatomic liquids (see refs. 28-35 and §§ 1.2.3, 2.10, and 4.10), and are usually taken into account, if at all, by an effective pair potential. There have been, especially at short range, relatively few theoretical evaluations of the pair potential for diatomic or polyatomic molecules (see, e.g. refs. 21-7 and 40-55a). The most reliable existing knowledge has been obtained from binary collision experiments, or, for the longrange part of the potential, from measurements of properties of single molecules. Examples include molecular beam scattering, induced birefringence, pressure and dielectric virial coefficients, and collision-induced absorption (including gas dimer spectra, which can also be studied by beam resonance spectroscopy) which yield values for the parameters (e.g. Lennard-Jones constants, polarizabilities, dipole moments, quadrupole moments, octopole moments, etc. - see also Appendix D) occurring in the expressions for the intermolecular potentials. The shape of the repulsive core of the potential can be inferred approximately from the molecular structure and charge density as determined experimentally, for example by electron and X-ray diffraction or by quantum calculations. As an example of the last point we show in Fig. 2.1 a contour map for the theoretically calculated charge density of N2 , the prototype molecule for simple nonpolar molecular fluid studies. Over 95 per cent of the total electronic charge is contained within the outermost (0.002 au) contour, and the dimensions of this contour are sometimes used to define a theoretical size of the N2 molecule. The dimensions shown on Fig. 2.1 agree roughly with dimensions obtained experimentally from Lennard- Jones diameters in gases (virial coefficients and viscosity) and so-called van der Waals radii from X-ray diffraction studies of solids.