Krzysztof Szalewicz of the University of Delaware is supported by an award from the Chemical Theory, Models and Computational Methods program to perform theoretical investigations of molecular clusters, condensed phases, and biochemical aggregates. The goal of this work is to increase understanding of physical, chemical, and biochemical phenomena that depend on inter-molecular forces. These properties can be predicted theoretically if high-accuracy potential energy surfaces, or force fields, are known. Such surfaces are computed ab initio using symmetry-adapted perturbation theory (SAPT) co-developed by the PI. 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. Such interpretation also provides a connection between the cluster properties and the properties of the constituent monomers. A version of SAPT based on monomers described by density-functional theory is currently the only method which can be applied to compute interactions between molecules containing about 100 atoms with uncertainties of only a few percent. SAPT is undergoing further developments, in particular applying localization techniques that should further increase the range of tractable systems. Several applications intended to assist interpretation of experimental data for systems ranging from diatoms to molecular crystals are underway. Such applications include developments of system-specific force fields which are then used in nuclear dynamics calculations and molecular simulations. Also, system-universal force fields are developed.
The proposed activity will result in a better understanding of intermolecular interactions which govern properties of condensed phases and biological aggregates. Novel ideas aimed at improvement of theoretical tools for investigations of these interactions are investigated. The development of first-principle-based universal force fields has the promise of transforming the field of biomolecular simulation. This activity will also result in new pressure and temperature standards, help in developments of "green chemistry", aid explorations of energy resources, provide data for constructing models of the atmosphere, help to understand processes in interstellar molecular clouds, guide development of nanomaterials, make simulations of biological phenomena more reliable, and assist pharmaceutical industry in predictions of polymorphic forms of medical drugs. The SAPT software available free to all interested researchers and used by about 500 groups worldwide is continuously improved in efficiency and user friendliness.
