Introduction

In a dilute gas, the molecules are widely separated and the molecules move several (tens) of angstroms before "colliding" with another molecule. In a liquid, the packing is very similar to that of a solid except that the molecules are moving all the time. In a solid, the molecules are fixed and the spacing between the molecules can be determined by X-ray diffraction. This will be taken up in a later chapter. In a liquid too there is a definite structural arrangement in the vicinity of every particle and when we take an average over the structural arrangement of all the molecules, what we obtain is the structure of a liquid.

In this lecture, we will characterize this structure and compare it with the structure of solids and gases. The functions characterizing the liquid structure are the spatial distribution functions (similar to the charge densities in atoms and molecules). We will study how these functions are determined by computer simulations. We will consider an explicit time evolution of a three particle system which illustrates the principles of classical molecular dynamics which is used not only in chemistry but also in a number of problems in engineering and molecular biology.

Before proceeding further, an important clarification needs to be made. The structure (of gases, liquids and solids) that is referred to above is not electronic structure, but the relative arrangement of molecules in space. In solids, these molecules are fixed in space and in liquids and gases, they are continuously moving. Electrons however are moving all the time and a "stationary" electron violates the uncertainty principle. The electronic structure of solids (band structure) is considered in a later module.

To describe the electronic structure of liquids, we need to describe the charge densities of moving nucleii. This is at present impractical. The major goal in this lecture is to relate the structure of liquids to intermolecular forces. Before doing so, let us see how the structure of liquids can be quantified.