The book deals with the prediction of the macroscopic behavior of fluids from the properties of their molecular constituents. The basis of such prediction is the availability of molecular models. Designing molecular models for fluids has an interdisciplinary background of foundations. Their thorough understanding is the basis for developing new models and appreciating the promise as well as the limitations of those that are established.

Molecular models are formulated in terms of the energy of a system of three-dimensional flexible bodies, i.e., the molecules. This molecular energy depends on the geometrical structures of the molecules and the force field they are moving in. The relation between the geometrical structure of a body and its energy is defined in mechanical terms. The force field results from the electrical properties of the molecules. On this level the models are thus based on classical mechanics and electrostatics. Classical theory, although most powerful even on the molecular scale, is, however, incomplete in the sense that it does not provide information on the geometry of the molecules or on their charge distributions as the origin of the electrical force field. This gap is closed by quantum mechanics, which also gives important corrections to the classical results in order to make them ultimately applicable to molecules. The link between the molecular energy of a system and its macroscopic thermodynamic functions is provided by statistical mechanics and by computer simulation. By using this link the molecular model leads to numerical data for the thermodynamic functions, from which the macroscopic behavior of a fluid can be calculated by the laws of classical thermodynamics.