The theoretical Chemical Physics program at Ames Laboratory supports integrated efforts in electronic structure theory and non-equilibrium statistical mechanical & multiscale modeling. The primary focus is on the development and application of methods that enable the study of surface phenomena, heterogeneous catalysis, cluster science and nucleation theory, and mechanisms in organometallic chemistry.
Electronic structure theory efforts involve PIs Mark Gordon, Klaus Ruedenberg and Theresa Windus and staff scientists Laimutis Bytautas and Mike Schmidt. These integrate development of fundamental theory (expanding the capability for accurate treatment of large or complex systems of interest to DOE), with optimal strategies for computational implementation within GAMESS and NWChem. A collaboration between Gordon and Ruedenberg and Ho and Wang (AL Condensed Matter Physics – CMP Program) is providing a rigorous basis for tight-binding methods which are of great use for studies of a diverse variety of surface and materials systems.
Efforts in non-equilibrium statistical mechanical and multiscale modeling of surface reaction phenomena by PI Jim Evans and staff scientist Da-Jiang Liu are often integrated with electronic structure studies in a close collaboration with Gordon. In studies related specifically to surface structure, Evans collaborates with Tringides, Hupalo, Ho and Wang (AL Condensed Matter Physics program and DOE CMSN), and both Evans and Liu collaborate with Thiel and Jenks (AL Materials Chemistry Program). Our modeling of heterogeneous catalysis also relates to experimental chemisorption studies by Thiel (AL Materials Chemistry Program).
Efforts by Gordon on organometallic systems and on metal oxide surfaces are largely related to collaborations with experimental colleagues Angelici, Pruski and Bakac (AL Heteroatom Catalysis Program), and separate electronic structure and statistical mechanical modeling by Gordon and Evans relates to studies by Lin, Pruski, Bakac and Sadow (AL Selective & Efficient Catalysis in 3D Controlled Environments Program – abbreviated as 3D Catalysis). Gordon also collaborates with Battaglia and Fox to determine the complex kinetics and mechanisms for the chemical vapor deposition of silicon carbide, starting from the methyltrichlorosilane precursor (NERI project).
A few highlights are included in the following paragraphs. Detailed information on any of these topics can be obtained from the links below to the principal investigators' web pages.
Atomistic and multi-scale modeling for catalysis and other surface reaction processes
Realistic atomistic (statistical mechanical) models have been developed for catalytic surface reactions such as CO-oxidation on metal(100) surfaces. These incorporate molecular-level information on interactions and ordering of adsorbed reactants, as well as on adsorption-desorption and reaction dynamics and energetics. These models are applied to assess bifurcations of reactive steady states, temperature-programmed reaction, novel phenomena for higher pressures and in fluctuation-dominated nanoscale reaction systems (such as supported metal catalysts). Heterogeneous multi-scale techniques (e.g., HCLG) are being developed to connect these models to mesoscale spatial pattern formation. Other work uses both atomistic and coarse-grained modeling to describe, e.g., etching processes which couple reaction to complex surface morphologies. These modeling efforts are integrated with ab-initio electronic structure studies (e.g., embedded cluster, DFT) which provide guidance to key energies and barriers.
Quantum mechanical electronic structure theory of chemical reactions
A major focus is the prediction of accurate energies along reaction paths on potential energy surfaces. Transition states, reaction mechanisms and conical intersections are among the features of particular interest. In this context, methods are developed for highly accurate quantum mechanical treatments of electron correlation and their quality is assured by reproducing experimental spectra of small molecules with diverse electronic structures. For the description of larger systems, methods of embedding high accuracy methods within zones of simpler methods are developed.
Other work is aimed at lowering the communication hurdle between quantum-chemical theorists and experimental chemists that exists because the functional concept of molecules formed by bonded atoms, validated by two hundred years of experimentation, does not emerge trivially from the basic physical theory of electrons and nuclei. The rigorous ab-initio quantification of the chemical model from physical theory is the focus of current work on the localization of molecular orbitals, the rigorous quantitative identification of atoms in ab-initio molecular wavefunctions and the resolution of ab-initio binding energies in terms of interpretable intra- and inter-atomic interactions.
Principal Investigator: Mark S. Gordon
Principal Investigator: James W. Evans
Principal Investigator: Klaus Ruedenberg
Principal Investigator: Theresa Windus