Krishnan Raghavachari is supported by an award from the Theoretical and Computational Chemistry program to develop theoretical methods for studying surface chemistry by developing and applying a set of hierarchical theoretical models. The hierarchical concept involves the use of highly accurate methodology on small systems to validate more approximate and computationally cost effective methodologies for investigating complex systems. Building on prior work in the PI's research group, Raghavachari and coworkers are developing more sophisticated and broadly applicable methods in three areas: (1) accurate thermochemical models applicable for 3d and 4d transition metal elements and fourth-row main group elements. These methods are based on the successful Gaussian-n (Gn) models, but will include many improvements to overcome known deficiencies; (2) divalent pseudoatom parameters to enable accurate and efficient calculations for investigating chemistry on (100) surfaces of tetrahedrally bonded systems; (3) new electronic embedding methods for an accurate treatment of QM/QM multilayer calculations based on the popular ONIOM framework. These developments are applied to a range of applications on semiconductor and metal-oxide surface chemistry, including (1) controlled functionalization of Si(100) surfaces, (2) alloy growth of compound semiconductor surfaces, (3) reactions of biomolecules on silica surfaces, and (4) catalytic mechanisms of titanium oxide and vanadium oxide surfaces.
The computer codes developed in this program are available to the scientific community as part of the Gaussian software package as well as in a set of freely available codes on the PI's website.
Computational modeling using quantum chemical methods is becoming increasingly important in chemical research. Over the past two decades the applicability of quantum chemical techniques has been pushed from the small molecule regime to larger and larger systems. Even with these advances significant challenges remain for the accurate treatment of complex systems. Intellectual Merit: In work carried out with this project, we have developed a set of new hierarchical computational models to investigate problems in surface and materials chemistry with more accuracy than possible previously. In particular, we have included important physically motivated effects in accurate calculations, and developed new methods that effectively treat electronic embedding and charge redistribution effects. Our methods have been applied to investigate the technologically important research area of dye-sensitized solar cells. We have also developed methods to derive parameters for "divalent pseudoatoms" that have enabled accurate and efficient calculations for investigating semiconductor surface chemistry. New methods have also been developed that can perform accurate calculations on the chemical reactions of adsorbates on surfaces using small and computationally efficient unit cells. Finally, a range of practical methods have been developed using a general fragment-based approach that makes it possible to treat large systems such as complex biomolecules with high accuracy. Our methods have been used in applications ranging from semiconductors to amino acids and peptides. Broader Impact: Our new developments fill a critical need in computational chemistry, providing a systematic well-tested models that are suitable for the study of complex problems in materials systems. We expect our models to be used by a broad range of investigators around the world. More than 30 scientific papers have been published in the open literature to benefit the chemistry community. Our research has also provided an excellent opportunity for training the next generation of researchers in the active and exciting field of computational chemistry. The work has also contributed significantly to the teaching in the graduate quantum chemistry course by including forefront examples from surface science applications.
