Professor Krzysztof Szalewicz of the University of Delaware is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to study forces acting between molecules. Although such forces are about ten times weaker than the chemical bonding forces acting inside molecules, they determine properties of most materials and living organisms. Intermolecular forces can be calculated using methods developed by Szalewicz and coworkers. A global analytic function fitted to results of such calculations is called its force field. Once the force field is known for a molecular cluster, condensed phase, or biological aggregate all properties of such systems can be predicted by solving the classical or quantum equations for atomic motions. One goal of this research is to increase the understanding of the dependence of these properties on specifics of intermolecular forces, which provides guidance in designing new materials. Another goal is to improve algorithms and computational methods to make first-principles predictions for systems several times larger than it is now possible. The physics-based force fields to be calculated hold promise for transforming the field of biomolecular simulations and of materials modelling and design. Such force fields are also relevant for metrology standards and for interpreting spectroscopic or scattering measurements, provide data for constructing models of the atmosphere, and help to understand processes in interstellar molecular clouds. The new crystal structure predictions methods can assist pharmaceutical industry in finding polymorphic forms of drugs.<br/><br/>The overarching goal of Szalewicz's research is to increase the first-principles-based understanding of physical, chemical, and biochemical phenomena that depend on intermolecular forces. High-accuracy intermolecular force fields can be computed using symmetry-adapted perturbation theory (SAPT), which was co-developed by Szalewicz's group. SAPT provides a unique ability to interpret properties dependent on intermolecular forces in terms of the four fundamental physical mechanisms that lead to the electrostatic, exchange, induction, and dispersion contributions to the interaction energies. SAPT based on monomers described by density-functional theory (DFT), a method denoted as SAPT(DFT), is as accurate as SAPT, but computationally more efficient. The developments of theory include work on nonlocal correlation functionals which predict accurate dispersion energies within the DFT framework, improved DFT methods that can be paired with accurate dispersion energies, extensions of the automated force-field generation method developed earlier in Szalewicz's group to flexible monomers, multicomponent methods for crystal structure predictions from first principles, and universal force fields based on ab initio computed monomer properties, a step in the direction of physics-based biomolecular force fields. Several applications of these methods to systems of current experimental, observational, or technological interest are also pursued.<br/><br/>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.