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Life in the Fast Lane

EMSL Scientists Accurately Calculate Charge Transfer State Across Spiro Molecule

Spiro cation molecule.

When a motorist drives from one city to the next, the most efficient route takes her on the freeway while avoiding wrong-way exits, dead-end streets and parking lots. The same is true for charge transfer of electrons across molecules. The challenge for scientists is mapping the fast lanes.

“In molecular systems, electrons move along potential energy surfaces that must be characterized very carefully if you are going to create a reliable computational model that predicts how the electron will move from one side of the molecule to the other,” explained Karol Kowalski, a senior research scientist at EMSL, the Environmental Molecular Sciences Laboratory. “You need a very accurate description of the electron freeway.”

Such a roadmap can lead to more effective applications of ultrafast molecular logic gates, molecular switches, molecular wires, and organic photovoltaic-based solar cells, but it requires a detailed study of the applicability of varied theoretical approaches of charge transfer processes in mixed-valence compounds.

Karol Kowalski and fellow EMSL scientists Kurt Glaesemann and Niri Govind, with Pacific Northwest National Laboratory (PNNL) scientist Sriram Krishnamoorthy, took on this challenge. They applied theoretical approaches in a supercomputing environment with the objective of building and validating new computational tools that quantum chemists can use to analyze the complex processes involving electron transfer.  A more complete understanding of charge transfer processes gives scientists the confidence to search new directions.

How they did it: EMSL researchers tested and validated various theoretical approaches with the objective of improving the capabilities of NWChem, the Department of Energy’s premier massively parallel computational chemistry user program, developed and maintained by EMSL.

The EMSL team began its research by selecting a challenge that stretched the limits of all theoretical approaches – analyzing potential energy surfaces of the Spiro cation molecule. This molecule has been the subject of recent intensive studies using high-level theory, including complete active space perturbation theory (CASPT2) and versions of n-electron valence state perturbation theory (NEVPT). Using EMSL computational capabilities, the team compared and contrasted three different approaches: multireference perturbation theory (MRPT), equation-of-motion coupled cluster theory (EOMCC), and time-dependent density functional theory (TDDFT). This was the first study of this molecule analyzing all three approaches.

Each approach presented opportunities, and each posed difficulties expressing the molecule’s potential energy surfaces. The team found that EOMCC – the most expensive approach – proved to be the most accurate. Ten years ago, for example, this theory was applicable only to small systems of 10 to 15 atoms. With the development of new efficient algorithms and today’s supercomputers, such as EMSL’s Chinook, larger systems of 85 atoms can be easily calculated. The team also used this “gold standard” approach as a metric to evaluate newly developed range-separated exchange-correlation functionals in less expensive approaches, like TDDFT.

With a fundamental understanding of charge transfer, new commercial technologies can be created and existing technologies improved. For example, the efficiency of harvesting solar energy can be greatly enhanced. Current technology is based on costly semiconductor solar cells. Long-chain hydrocarbon organic polymers, meanwhile, are inexpensive to manufacture. With accurately calculated charge transfer, they may represent a new generation of highly efficient, low-cost photovoltaics. Other applications may include super-efficient light-emitting diodes (LED) and commercial applications for carbon graphene, a single-atom-thick planar sheet of carbon atoms bonded in a honeycomb lattice.

What’s next? With theoretical approaches compared and EOMCC shown to accurately calculate the charge transfer state, researchers can now use this approach to validate and improve less costly approaches, such as TDDFT, to calculate larger molecular systems. 

Scientific impact: The integration of this highly accurate theoretical approach in EMSL’s NWChem software gives researchers a powerful tool to better understand the various processes that happen when electrons move across molecules.

Societal impact: With a better understanding of charge transfer, new commercial technologies can be created and existing technologies improved to offer greater efficiency and performance at lower costs.

Reference: KR Glaesemann, N Govind, S Krishnamoorthy, K Kowalski. 2010.  ‘EOMCC, MRPT, and TDDFT Studies of Charge Transfer Processes in Mixed-Valence Compounds: Application to the Spiro Molecule.’  Journal of Physical Chemistry A (114 (33), pp 8764–8771) A. doi:10.1021/jp101761d.

Acknowledgments: This research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research. All the computations were performed on EMSL’s supercomputer, Chinook. This work was also supported by the Extreme Scale Computing Initiative, a Laboratory Directed Research and Development Program at PNNL.

 

Released: September 29, 2010