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J. Woods Halley, Fellow

Numerical Studies of Fluids and Disordered Solids

Research Group

Sarah Alfano, Supercomputing Institute Undergraduate Intern
Bach Thanh Cong, Solid State Physics Department, Hanoi University of Science, Hanoi, Vietnam
Yuhua Duan, Research Associate
Justin Hietala, Supercomputing Institute Undergraduate Intern
Robert Joynt, Physics Department, University of Wisconsin, Madison, Wisconsin
Keith Lidke, Graduate Student Researcher
You Lin, Graduate Student Researcher
Brooke L. Nielsen, Undergraduate Student Researcher
Amanika Prasad, School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota
David L. Price, Morgantown, West Virginia
Patrick Schelling, Argonne National Laboratory, Argonne, Illinois
Arun Setty, Department of Physics, Purdue University, West Lafayette, Indiana
Sean Walbran, Forschumgzentrum Julich, Julich, Germany
Aaron Wynveen, Graduate Student Researcher
Min Zhuang, Research Associate

Atomically relaxed titanium/titanium dioxide interfaces (bottom) obtained using self-consistent tight binding simulation techniques.

In this ongoing program, simulation methods are used to study condensed matter interfaces. Simulations of the electrode-electrolyte interface include the dynamics of the molecules during electron transfer to or from the electrode, and the study of the electronic structure of oxides and metals at the electrode-electrolyte interface using self-consistent tight binding molecular dynamics as well as ab initio plane wave techniques. These researchers are studying solvation, transport and ion pairing in polymer electrolytes and their instances with battery electro density molecular dynamics techniques. Finally, the interaction of gases with superfluid helium four and dilute Bose-Einstein condensed systems.

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Calculated activation energy, transfer coefficient, and equilibrium potential of electron transfer of the copper water interface have been found to be consistent with experimental results obtained by collaborators at Argonne. Former graduate student, Sean Walbran,now a Humboldt fellow at the Max Planck Institute in Julich, Gemany, obtained these results on the cuprous-cupric electron transfer reaction in water at a copper electrode. Qualitatively, the copper electron transfer reaction differs considerably from this group's previous study of ferrous-ferric electron transfer-in the copper case, the electrode approach energy dominates the reaction barrier, whereas in the iron case, solvent fluctuation dynamics were found to be the main contribution.

For studies of inner shell-electrochemical processes, simultaneous calculation of metallic electronic structure and atomic dynamics (direct dynamics) is required. These studies first focussed on a simple model for a copper electrode in contact with water. Postdoctoral research associate, Yu Zhou, modeled the interaction of a chloride ion with a copper surface using modifications of this code. Walbran implemented non-local pseudo-potentials in this model for the study of aluminum and magnesium electrodes, obtaining work functions, cohesive energies, and structural parameters in agreement with experimental values. Collaborator David Price has also completed a calculation of the Cd-H2O interface. These researchers are now extending these studies of s-p metals. In particular, they are now able to allow the electrode atoms as well as solvent molecules to relax in these simulations. However, Walbran, in his thesis, showed that a full treatment of the d-electrons on the copper electrode will be computationally infeasible for some time.

The current program for attacking these more complicated systems involves an extension of earlier work on tight binding models of oxides that initally used equation of motion techniques to make one electron tight binding studies of defects in rutile titanium dioxide. More recently, these calculations were made Hartree self-consistent, included atomic motion, and fit the model to bulk results from first principles calculations in collaboration with the group of James Chelikowsky of the Chemical Engineering and Materials Science Department at the University of Minnesota. A study of localized electronic states (polarons) in bulk rutile as a function of temperature and box size has been completed using this code, and results have been obtained that demonstrate that localization occurs at temperatures above 100 K. Codes have been written to implement order N algorithms for these methods that have also been used in collaboration with Argonne National Laboratory to study the structure of twist grain boundaries in rutile. During the last period, this work has found that electronic structure of transition metals can be surprisingly well represented by the same methods. Patrick Schelling, a former graduate student and now a postdoc at Argonne, has made such models for titanium and copper. He is currently studying a model of the titanium/rutile interface using these methods.

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