Chapter 3 is devoted to realistic interatomic potential energy functions, and begins with a discussion of the need for more accurate representations of these functions. Direct inversion of both microscopic data (spectroscopic transition frequencies and atomic beam scattering) and bulk property data (pressure and acoustic second virial coefficients, transport properties) are discussed in detail. The quantum chemical ab initio determination of binary atomic interaction energies and their analytical representation are discussed, followed by a detailed considerations of the interaction energies between pairs of noble gas atoms. Chapter 4 is concerned with connections between theory and experiment, including a detailed discussion of pure noble gases and their binary mixtures. Chapter 5 focuses on how to obtain the spectroscopic and thermophysical properties of a specific molecular system theoretically step by step, and provides a reference for the specific theoretical calculation work.