Theoretical Studies of Intermolecular Forces

University Of Delaware, Newark DE

Investigators

Abstract

With support from Chemical Theory, Models and Computational Methods (CTMC) program in the Division of Chemistry, Professor Krzysztof Szalewicz of University of Delaware is performing quantum-mechanical investigations of clusters of molecules, molecular condensed phases, and biomolecular systems. The properties of such systems are governed by intermolecular (van der Waals) forces: depending on the distance between two molecules, they will either attract or repel each other, a physical law that Richard Feynman considered to be the biggest finding of humanity. Szalewicz and coworkers have developed methods for computing intermolecular forces that are not only among the most accurate and computationally efficient ones, but also provide researchers with a unique ability to interpret properties dependent on intermolecular forces in terms of the four fundamental physical mechanisms: the electrostatic, exchange-repulsion, polarization, and dispersion interactions. Since direct quantum-mechanical calculations are limited to molecular assemblies with a hundred or so atoms, Szalewicz’s group will develop machine-learning methods of extrapolating quantum results to arbitrary-size systems. The importance of this work stems from its ability to predict properties of matter from first principles, i.e., deriving them from equations of quantum mechanics, for arbitrary molecular materials. One example are reliable predictions of crystal structures. Computational design of crystals is of significant importance for pharmaceutical, agrochemical, semiconductor, and energetic materials industries. The fundamental research on intermolecular forces being conducted here has the potential for broad scientific impact across a wide array of fields from materials science to biomolecular, and atmospheric science, to molecular spectroscopy and astrophysics. Broader impacts of this activity include training of graduate students and postdoctoral associates with diverse backgrounds, extensive collaborations with other research groups, organization of conferences and workshops that will disseminate knowledge, participation in award committees and in activities of an international academia, and developments of free software to be used by other researchers. Under this ward, the Szalewicz group is developing and utilizing symmetry-adapted perturbation theory (SAPT) based on monomers described by density-functional theory (DFT), an approach denoted as SAPT(DFT). Machine-learning methods for the generation of force fields derived from SAPT(DFT) calculations will be extended to enable treatment of molecules with soft internal degrees of freedom. These force fields will be used for several systems of current experimental, observational, or technological interest, in particular for predictions of crystal structures from first principles, including difficult cases with polymorphism related to varying conformations of monomers. Other developments of theory will include work on nonlocal-correlation DFT methods that can reliably predict dispersion energies, improved DFT methods that can be paired with accurate dispersion energies, extensions of machine-learning force-field generation methods to three-body nonadditive interactions, and universal force fields based on ab initio computed monomer properties, a step in the direction of physics-based biomolecular force fields. These studies are expected to advance knowledge of the field of intermolecular interactions and to have promise for transformative changes in electronic structure methods, in force-field development techniques, and in crystal structure predictions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.