IL NUOVO CIMENTO VOL. 12 D, N. 4-5 Aprile-Maggio 1990 A Theory of Fluids Reliable in the Critical Region (*). A. PAROLA S.I.S.S.A. - Strada Costiera 11, Trieste, Italia A. MERONI and L. REATTO Dipartimento di Fisica dell'Universitd - Milano, Italia (ricevuto il 13 Ottobre 1989) Summary. -- A theory for the build-up of long-range correlations in fluids is developed. When it is supported by a realistic description of the short-range structure, based on the optimized random phase approximation, it shows the typical accuracy of a good liquid-state theory far from the critical point together with a genuine nonclassical critical behaviour. In this way we obtain a comprehensive theory of fluids in which both universal and nonuniversal quantities can be studied for a realistic model of a liquid. The case of the Lennard-Jones interaction is analysed in detail above the critical temper- ature and is compared to experiments and numerical simulations. PACS 61.20 - Classical, semiclassical, and quantum theories of liquid structure. PACS 64.70 - Phase equilibria, phase transitions, and critical points of specific substances. The main concern of liquid-state theories is the description of the local arrangements of molecules, because this is by far the most relevant feature in the dense region. In fact the structure of a simple fluid, at moderate and high density, is mostly determined by excluded volume effects induced by the short- range repulsive interaction, while the attractive contribution gives rise to a smoothly varying background potential which provides the cohesive energy of (*) Paper presented at the workshop ,,Highlights on Simple Liquids,, held in Turin at ISI on 1-3 May, 1989. 29 - II Nuovo Cimento D 419