"Scientist have been trying to design efficient, durable devices that can functionally mimic light harvesting processes as they happen in nature", stated Rajendra Zope, a professor of physics at the University of Texas El Paso (UTEP).
Breakthroughs in nanotechnology and nanostructured materials have led to artificial photosynthetic systems that can convert light into usable energy.
In the 1990s, researchers developed a molecule called PCBM - Phenyl-C61-butyric acid methyl ester - a hollow cage of carbon atoms derived from a 'buckyball', that could transform sunlight into electrical energy. However a fully-functional, scalable and stable form of organic artificial photosynthesis system has yet to emerge.
For the development of functional materials, it is necessary to understand the photoconversion process taking place inside the material. In this area, supercomputers play a critical role by allowing researchers to study materials' properties, such as their quantum electronic structure, and by providing insights into how, where and when electrons move through the active material using simulations.
One of the challenges in simulating complex systems like artificial photosynthetic molecules is the size of these entities, which researchers try to model from the ground up - also known as 'ab-initio'. Another challenge lies in trying to pinpoint the charge transfer excited states in detail.
For several years, Rajendra Zope and his colleagues in the UTEP Electronic Structure Lab have used the Stampede and Lonestar supercomputers at the Texas Advanced Computing Center (TACC) - some of the most powerful in the world - to model the electronic structure of artificial photosynthetic systems.
Writing in the journal Chemical Physics , they described calculations related to a photosynthetic, multichormophoric antenna system comprising of 421 atoms that is able to absorb light from many different wavelengths.
"Our simulations are done to understand the energy level ordering of the system", he stated. "The ordering is very important to see if the charge transfer exciton will deliver the current or not."
In addition to studying the ordering, they explored how the inclusion of various ligands - molecules bound to a metal atom - changed the dynamics of the system, making it more or less effective at transforming light into energy. They also predicted the charge transfer energy of the molecular complex in various configurations.
The work is supported by grants from the Department of Energy and the National Science Foundation.
Before Rajendra Zope and his collaborators can run their simulations, they must first develop the computer code capable of mathematically modelling the quantum energetics of systems as big as the ones they study.
The software they develop and maintain for this purpose is known as NRLMOL - the Naval Research Laboratory Molecular Orbital Library. A massively parallel code for electronic structure calculations on large molecules and clusters, it is used by more than a dozen groups around the world to study not only light harvesting systems, but also molecular magnets and transitional metal systems - both promising systems for next-generation nano-electronics.
Creating effective algorithms entails using a large number of computer processing cores concurrently while minimizing the memory usage on every processor.
"There are very few codes that scale as well as our code in terms of the number of cores it can use", Rajendra Zope stated. "For the excited state calculations of the light harvesting hexad containing 421 atoms, we can use up to 1,000 processors with 70 percent parallel efficiency. This is quite good for density functional electronic structure codes like NRLMOL that use Gaussian basis sets."
Rajendra Zope and his team have used TACC systems since 2009. They were first able to access TACC systems through the University of Texas Research Cyberinfrastructure (UTRC) programme, which makes TACC's computing resources, expertise and training available to researchers within the University of Texas Systems' 14 institutions.
They also access TACC systems through the Extreme Science and Engineering Discovery Environment (XSEDE), which allocates time to researchers on national supercomputing resources. In addition to TACC supercomputers, Rajendra Zope's team uses systems at the U.S. Department of Energy's National Energy Research Scientific Computing Center.
Light harvesting research has been advancing in recent years, but has yet to reach the level of widespread commercial use. However, Rajendra Zope believes that in a decade, many of the fundamental questions about light harvesting nanostructures will have been worked out and the area will blossom.
"It would have a tremendous effect", Rajendra Zope stated. "Per person, energy use is continuously growing and the population is growing, so the energy demand is significant. Once we figure out the mechanism to improve efficiency, then all we are doing is harnessing the light. If we realize this, we will have solved the energy problem in a nice, clean manner."