UC San Diego

Chemical systems are inherently difficult to model due to their many-body nature. Difficulties associated with the large number of degrees of freedom are further com- plicated by the need for a quantum mechanical treatment of the electrons (and the nuclei in many circumstances). Here we employ various types of decompositions in order to dissect the interactions and dynamics in chemical systems, thus facilitating the development of models that can be used in molecular dynamics simulations, as well as facilitating the analysis of the resulting simulations. In particular, the many-body expansion is used to study the intermolecular interactions in protonated and deprotonated water clusters, as well as alkali ion-water clusters. The many-body energies of protonated and deprotonated water clusters are decomposed into contributions due to so-called fragment groups which are defined by their hydrogen bonding connectivity. Fragment group energies are further decomposed using energy decomposition analysis to investigate the physical interactions that lead to cooperative and anticooperative hydrogen bonding. Additionally, the concept of a fragment group is extended in order to derive scaling relations for many-body interactions in lattices. These investigations of many-body interactions are used in the development and evaluation of model potential energy functions describing intermolecular interactions. Molecular dynamics simulations of the Zundel ion dissolved in liquid acetonitrile, using these many-body potential energy functions, are analyzed with the help of decompositions of the vibrational power spectra. First steps in the development of many-body valence bond models describing aqueous proton transfer are also discussed.