Theorectical Studies of van der Waals Molecules

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This project, supported by the NSF Theoretical and Computational Chemistry Program, investigates the phenomenon of intermolecular interactions. Prof. Krzysztof Szalewicz of the University of Delaware and collaborators will further improve symmetry-adapted perturbation theory (SAPT) developed by them for dimer interactions, and a new version applicable to trimers will be derived. This theory allows transparent physical interpretation of intermolecular potentials in terms of four fundamental interactions: electrostatic, exchange, induction, and dispersion. The theory is free from basis set superposition problems and is computationally more efficient than the supermolecular many-body perturbation theory approach. High-level calculations of state-of-the-art intermolecular potential energy surfaces for experimentally relevant van der Waals dimers and trimers will be performed. These studies will help provide an understanding of the real physical mechanism for the formation of van der Waals bonds and the physical interactions that are responsible for minima, tunneling barriers, and anisotropies of the potentials. This work is expected to assist interpretation of experimental data, particularly in the case of three-body nonadditivities. An important part of the project will involve applications aimed at resolving current problems in the field concerning interactions of rare gas atoms with various molecules such as nitrogen, carbon dioxide, water, or ammonia, as well as dimers of those molecules. The trimers will include rare gas atoms, Ar2-HF, and the water trimer. The current theoretical understanding of three-body interactions is very unsatisfactory, in particular in view of recent experimental activity in this field. The SAPT method offers the possibility of physical analysis of nonadditivities and of obtaining accurate numerical values for them. This project investigates the phenomenon of interactions between molecules. These interactions, much weaker than the chemical forces binding atoms in molecules, are responsible for an extensive range of physical, chemical, and biological phenomena: from the boiling of water to the genetic code. Standard computational chemistry methods, which are very successful in describing chemical bonds, are inadequate in describing these intermolecular forces. Szalewicz and collaborators will develop and use rigorous methods based on a quantum mechanical description of these systems in order to provide a more complete understanding of these molecular interactions.