Vibrational spectroscopy is among the main tools of physical chemistry in the exploration of molecular properties. Interpretation of the experiments requires theoretical calculations of the spectra, and for large, non-rigid molecules this is a formidable challenge. It implies calculations of the wave functions an energy levels of many interacting vibrational degrees of freedom. An algorithm developed in our group in recent years, the Vibrational Self-Consistent Field (VSCF) method(1), led to major progress on this problem, and has emerged as a leading tool in this field. Comparison with experiments, using this computational method, has already led to the unravelling of properties of biological molecules, large hydrogen-bonded clusters and other very "floppy" molecular systems. An early application was the calculation of the vibrational spectroscopy of the protein BPTI(2), using an available force field for this system. This predicts features of protein spectroscopy, that can guide future experiments. Recently, we were able to develop an algorithm that combines ab initio electronic structure codes with the VSCF method(3). This provides a method that computes directly vibrational spectroscopy from electronic structure theory. The method was applied to biological molecules(4) and to large hydrogen-bonded complexes. It was found that ab initio potential energy surfaces are far superior to the available analytical force fields. Our research in this area continues towards investigation of peptides, proteins and nucleic acids.

Another, altogether different research field of our group is chemical reaction dynamics at very low temperatures. There have been dramatic recent developments in low-temperature chemistry of hydrides, e.g., HF in rare-gas solids was found to lead to formation of new and surprising compounds. HArF, the first chemically-bound compound of argon, was so prepared. We study the formation dynamics of novel rare-gas compounds, by simulations of the photochemical reactions in the rare-gas environments. An interesting prediction is of HHeF(5), a first chemically-bound compound of helium: We computed it to be metastable, with a lifetime of picoseconds as a free molecule, but to be stable for indefinite times in pressurized solid helium. Several other new types of rare-gas molecules were predicted in this research. The new molecules are unusual in bonding, reactivity and many other properties. These results have already impacted experiments substantially(6),(7), and our search continues for novel species and for exotic properties. The dynamical calculations must incorporate quantum effects at the low temperatures: The development of new algorithms for quantum dynamics of large systems(8) is thus a central goal of this research