An image from a simulation of the full dynamics of an electronic excitation created on a section of the polymer polythiophene. The purple spheres represent the location of the excitation, with radius scaling with the distribution of that excitation over the molecular rings.
This year’s Nobel Prize in Chemistry recognized computational modeling as a game-changer in molecular research.
Awarded to three American-based scientists, the prize acknowledged the impact multi-scale models, which couple quantum mechanics and classical physics into a single computational model, have had in enabling the simulation of the chemical activity in large complex molecules.
ICES’ Peter Rossky earned his Ph.D in chemical physics under Martin Karplus, one of this year’s Nobel laureates. Now, as director of the ICES Center for Computational Molecular Sciences, Rossky is advancing such multi-scale models toward understanding the behavior of molecular materials in a variety of ways, with his most recent research focused on developing plastic-based solar cells.
Applying coupled quantum-classical modeling toward solar cells is a relatively new venture for Rossky, who has spent a large portion of his career investigating how water influences the chemistry of molecules in a solution. This research began in Karplus’ lab, where the coupled model had been created just a few years earlier to examine proteins. Under Karplus, Rossky created the first computer simulation of a water-solvated protein, modeling the protein in an environment akin to what is experienced in living biological systems.
“Water is the medium in which biology occurs, so if you don’t know what the water is doing, you may not understand why [a protein] is functioning the way it does,” said Rossky.
Rossky did not use the combined model while in the Karplus lab, but it became a critical tool for his later research. He first applied it in the 1980s when studying hydrated electrons, a major player in radiation sickness because of their high reactivity. While his advisor focused on modeling biological molecules, Rossky is currently interested in the synthetic; the combined models are a key part in evaluating the molecular behavior of candidate photovoltaic materials for solar cells.
“The 1972 paper by Warshal and Karplus is the primary reference for the basic formulation,” said Rossky, referring to the publication that first described the coupled quantum-classical model.
Peter Rossky holds the Marvin K. Collie-Welch Regents Chair in Chemistry in the Department of Chemistry & Biochemistry, and is professor of chemical engineering. He also serves as the director of the ICES Center for Computational Molecular Sciences, and is a member of the ICES Multiscale Modeling Group.
The University of Texas at Austin’s Energy Frontier Research Center (EFRC) for Charge Separation and Transfer, which Rossky directs, sponsors solar cell and battery research, with a focus on understanding how energy capture and storage could be improved starting at the molecular level. ICES researchers Graeme Henkelman, Greg Rodin, and Venkat Ganesan are also involved in the EFRC’s efforts.
“Our EFRC is focused entirely on understanding the fundamentals by using systems that are well controlled. We know what molecules are there, but the goal is to characterize how they’re arranged and how they change with light absorption, so that we can learn what the fundamental roadblocks are and develop the principles that can be used as a basis for designing molecular components”, said Rossky.
The ultimate goal is to find a solar cell material that could be produced at low cost in lightweight, plastic, flexible sheets with high energy producing efficiency. While there are current plastic solar cell materials, they are still considerably less efficient and less durable compared to conventional silicon panels. (See a simulation of electron excitation in polythiophene, one candidate material)
Like the models created by the three Nobel laureates, Rossky’s molecular models are multi-scale, with active portions being modeled with quantum mechanics, and surrounding structure in classical, mechanical terms. Working with ICES’ Director Tinsley Oden, Rossky is taking these models to yet another level: the mesoscopic realm, the scale that describes the complex arrangement of large collections of molecules.
The efforts of contemporary research in computational molecular science are following the foundations recognized by the 2013 Nobel Prize in Chemistry while continuing to expand the scope of the vision accessible via simulation.
“What you see now is a very natural evolution of what’s been going on since the 1960s. There are continually improvements in both the quality of the description at any given scale and also the ability to reach over larger scales of time and larger scales of space,” said Rossky.
This article was originally published on The University of Texas Institute for Computational Engineering and Sciences (ICES) website.