This project in the physical chemistry program focuses on the theoretical determination of transient, interaction-induced changes in molecular dipoles and polarizabilities on the time scale of molecular collisions. Where overlap between the interacting molecules is slight, calculations will be based on the PI's nonlocal polarizability density model, which accounts for the distribution of polarizable matter throughout each molecule. The model will be extended to include a practical means of computing nonlocal, nonlinear response. The polarizability densities will be analyzed for physical content, and collision-induced properties of spectroscopic interest will be computed. The PI will also apply reaction field theory to find dispersion contributions to the polarizabilities of anisotropic molecules for the first time, and extend the theory to frequency-dependent response. To incorporate nonadditive exchange-polarization where overlap is moderate, the PI plans to perform density functional and exchange-perturbation calculations; the latter also include charge transfer. Required single-molecule moments, susceptibilities, and short-range pair properties will be determined by ab initio calculations. Representative species to be studied include H2...H2, N2...N2, Ar...Ar, and Ar...HCl. This research is important for its spectroscopic applications, since significant information on intermolecular dynamics can be extracted from collision-induced absorption and scattering spectra, if the two collision-induced dipoles and polarizabilities have been determined as functions of intermolecular distances, relative orientations, and vibrational frequency.
