Interactions of Atoms and Molecules in Cold Chemistry

We present the basic ideas about the broad topic of molecular interactions in the context of low-energy collisions. We start by introducing the concept of the interaction energy and the crucial approximation behind it—the Born–Oppenheimer approximation. Next, we focus on understanding of molecular interactions within the framework of the perturbation theory. We discuss the essentials of symmetry-adapted perturbation theory (SAPT) which provides partitioning of the interaction energy into physically meaningful components, such as electrostatic, dispersion, induction, and exchange interactions. We examine the role of these components and how they can be calculated. We also discuss the supermolecular approach to calculation of the interaction energy with various quantum chemistry methods. We indicate potential pitfalls of the latter approach and give recommendations on how to alleviate them. To illustrate the theory presented in this chapter, we provide a detailed discussion of interactions within several systems of relevance in cold chemistry and cold collisions: He-NH(3Σ−), N-NH, He-OH(2Π), NH-NH, O2(3Σ−g)-O2. The calculations of collision rates and scattering cross-sections impose stringent requirements on the accuracy of the potential energy surfaces. To address the issue, we present the analysis of the scattering length dependence upon the potential scaling. Finally, we indicate trends in calculations of intermolecular forces stimulated by the state-of-the-art cold chemistry experiments.