The Structure of Simple Liquids

The theory of liquids has been reviewed in three recent articles in this series; those of Neece & Widom (1), Berne & Forster (2), and Barker & Henderson (3). These articles describe how in recent years the theory of liquids has made substantial advances and many problems which have been tantalizing workers for the past ten or twenty years have been solved. The consequences of the pair theory of liquids have been worked out and reasonable comparisons made with experimental data on both model and real systems. In the case of the liquified inert gases the field is fairly healthy with general ideas having been successfully developed and fairly satisfactory calculations made in the case of many properties. For simple molecular liquids or liquid metals, the position is less satisfactory but steady progress is being made. Because of the background covered in these three articles the present author will assume the reader is familiar with basic liquid state theory, and he will express his own point of view on current theoretical and experimental situations rather than express an average view. In particular, emphasis will be placed upon the physical ideas involved and the basic merit of a method of calculation. That is, phenomenological treatments will not be regarded as sufficient even where they give exact agreement between calculation and experiment : the view taken is that it is better to obtain approximate agreement via a basic treatment rather than to obtain exact agreement with a phenomenological treatment. Recent methods of discussing the static and dynamic pair correlation functions, and some methods of calculating the diffusion and viscosity coefficients will be considered from the above point of view. The basic hypothesis on which the modern understanding of the behavior of simple liquids is based, is that the short range behavior determines the large range behavior. Thus we suppose that the short distance behavior determines the large distance behavior and the short time behavior determines the long time behavior, and that if we can make a successful "guess" at the short