Correlation Functions and the Critical Region of Simple Fluids

The ``classical'' (e.g. van der Waals) theories of the gas-liquid critical point are reviewed briefly and the predictions concerning the nature of the singularities of the coexistence curve, the specific heat, and the compressibilities are compared critically with experiment and with the analytical and numerical results for lattice gas models. The critical singularities are related to the behavior of the pair correlation function G(r) = g(r) − 1 and the Ornstein-Zernike theory of critical scattering is reviewed. Alternative derivations of the theory are discussed and its validity is assessed in relation to experiment and to more detailed theoretical calculations. The nature and magnitude of the expected deviations from the ``classical'' theory are described. The analogies with critical magnetic phenomena are mentioned briefly.