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Krzysztof Szalewicz



UD » Physics & Astronomy » Faculty » Krzysztof Szalewicz » Research

Intermolecular Forces

Intermolecular forces, i.e., forces acting between molecules, although much weaker than the chemical forces acting within a molecule, are responsible for the existence of liquids and solids as well as for most of biological processes including the DNA coding. Knowledge of these forces allows one to better explain various properties of condensed phase, for example why ice is less dense than water. Theory can predict in particular, without using any experimental information, how substances will behave under extreme conditions where experiments are difficult to perform. Our group has developed perturbative methods describing the phenomenon of intermolecular interactions, the so-called symmetry-adapted perturbation theory (SAPT). These methods solve the quantum mechanical Schroedinger equation using a sequence of gradually improving approximations. In contrast to the standard perturbation theory, the proper permutational symmetry is imposed on the wave functions in each step. This is necessary to describe the phenomenon of electron tunneling between the interacting systems resulting in the so-called exchange force. The electrostatic, induction, and dispersion forces are included as well. Investigations of model systems determined the convergence properties of the resulting expansions of interaction energies and wave functions and led to new expansions with more satisfactory convergence properties. The other important development was the introduction of the many-body techniques that allowed applications of SAPT to many-electron systems. Practical calculations are now possible for fairly large molecules (containing up to about ten atoms). The SAPT computer code written by our group is currently used by more than 70 research groups worldwide. This code has been included in the list of key programs by the DOD and the adaptation of this program to parallel computers has been supported by the DOD's Common High-Performance Computing Software Support Initiative (CHSSI) program. The SAPT method is under continuous development. One direction is to increase the accuracy of predictions and the other is to enable calculations for much larger systems than currently possible.The SAPT code has been used to investigate interactions in several systems of significant interests. In particular a SAPT interaction potential for helium is much more accurate than previously available ones. All phases of helium are of great interest in physics. The experimental observations involving helium can be predicted and analyzed by various theoretical methods, most of them utilizing the helium dimer interaction potential. The SAPT potential has been used to calibrate helium-based thermometry and transport properties standards and to determine the equation of state for superfluid helium. Another system studied recently was water. The water dimer and trimer have been extensively investigated by molecular beam spectroscopy techniques. Calculations based on the SAPT potentials for these systems predicted spectra that quantitatively agreed with experiment, while previous literature attempts were not able to even qualitatively describe the spectra. The potentials were also used to simulate the properties of liquid water. Such simulations can unravel the structure and dynamics of condensed water that determines the anomalous properties of this phase. Another project succeeded in prediction of the correct crystal structure of argon, the goal that eluded theory for fifty years. Numerous other van der Waals dimers have been investigated. These calculations predict the spectra so accurately that one can consider these to be as reliable as experimental determinations. SAPT potentials are used in simulations of several liquids and vapor-liquid equilibria.

Exotic molecules

Exotic systems are atoms and molecules containing ``exotic" particles such as muons or antiprotons. Theoretical investigations were instrumental in determining the feasiblity of using muons to catalyze nuclear fusion. While hydrogen isotopic nuclei spontaneously fuse in a hydrogen molecule less than once in the lifetime of the universe, if electrons are replaced by muons, the resulting exotic molecule is much smaller than the original one and fusion takes place in less than a trillionths of a second. More recently similar techniques have been applied to molecules containing antiprotons which are the media where in certain cases matter and antimatter can coexist for as long as a microsecond before annihilating. This research was conducted in collaborations with experiments performed at CERN.

Mass of neutrino

A past project was relevant to high energy physics and aimed at the determination of the mass of neutrinos, atomic particles so weakly interacting with matter that they easily flow through everything on Earth. Some scientists theorized that neutrinos could be foundation of the dark mass of the universe. Results of molecular calculations were used to interpret measurements which put strict limits on the mass of the neutrino showing that it is not large enough to contribute significantly to dark matter.

Electron correlation in atoms and molecules

The simplest description of an atom or molecule uses the concept of orbitals, i.e., the independent-particle model. The difference between the predictions of this model and the exact values are due to correlations of the motions of electrons. To describe such effects, one uses many-body perturbation theory and coupled cluster methods. Most implementations of these methods expand wave functions into products of orbitals which severly limits accuracy of predictions. This problem can be overcome by using explicitly-correlated functions, i.e., functions depending on interelectronic distances in the coupled cluster expansions of the molecular wave functions. For small molecules this method produces benchmark correlation energies which serve as accuracy standards for other theoretical methods and provide closest possible agreement with experiment.

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