Hyperspherical representation of potential energy surfaces: intermolecular interactions in tetra-atomic and penta-atomic systems

In this paper, we discuss the applications of spherical and hyperspherical harmonics expansions for potential energy surfaces for intermolecular interactions, focusing on four- and five-body problems. Case studies include diatomic-molecule-diatomic-molecule systems, floppy-molecule-rare-gas-atom cases, atom-triatomic, and nonlinear-molecule-linear-molecule systems. Among these systems, the important ones regarding water interaction with atoms and diatomic molecules are presented here to illustrate the use of spherical and hyperspherical coordinates and harmonics for weakly interacting systems. The interaction potential representation involves the distance between the centers of mass of the two molecules and a set of angles that defines the mutual orientation of the molecules, whose geometries are assumed to be either 'frozen' in their equilibrium configurations or to vary according to specific large-amplitude modes. The potential energy surfaces in the given examples of water-hydrogen, nitrogen and oxygen interactions are generated at the CCSD(T)/aug-cc-pVTZ level. Comparisons to the theoretical and experimental results of recent works for these and similar systems are presented.