NERSC Initiative for Scientific Exploration (NISE) 2010 Awards

Modeling the Energy Flow in the Ozone Recombination Reaction

Dmitri Babikov, Marquette University

Associated NERSC Project: Coherent Control of the Ground Electronic State Molecular Dynamics (m409), Principal Investigator: Dmitri Babikov

By resolving the remaining mysteries of what determines the isotopic composition of atmospheric ozone and by investigating unusual isotope effects in other atmospheric species, this work will make a large impact on the fields of Atmospheric Chemistry, Chemical Physics, Climate Science and Geochemistry. Explaining the anomalous isotope effects in O₃, NO₂ and CO₂ will significantly improve our understanding of their production, chemistry, lifetime and loss in the atmosphere. That knowledge will help to identify and remove pollution sources as well as monitor the ozone hole, with the possible impact on enhancing the security of all life on the planet. It will allow the isotopic composition of oxygen to be used as a reliable probe of its source and history and provide information for studying atmospheric chemistry, global climate change, atmospheres of other planets and the history of the solar system.

To do this, we will develop a massively parallel code that should allow us to model very efficiently a flow of ro-vibrational energy in atom-molecule collisions typical for many recombination reactions. Such chemical processes proceed through formation of an intermediate long-lived metastable state (scattering resonance).

We develop an efficient theoretical framework which should allow us to make this problem treatable computationally with emphasis on massive scaling. Our approach is to keep quantum mechanics for description of the vibrational motion in O₃* (using 3D-wavepacket formalism) but to treat the overall rotation of O₃* and the M + O₃* collisional motion using classical trajectories. In such a mixed quantum-classical approach the quantum physics of the process (zero-point energy, scattering resonances and symmetry rules for state-to-state transitions) is captured by the vibrational wavepacket, while the classical trajectory part of the system allows sampling initial conditions efficiently by running a set of independent calculations on different processors. For typical calculations, the number of classical trajectories needed is between 10,000 and 100,000, which can be propagated on different processors. With such approach we can easily employ thousands of processors simultaneously.

The fully quantum treatment of such processes is unaffordable due to poor scalability of the intrinsically global quantum mechanics, while the fully classical treatment looses all quantum physics of the process. It was shown, however, that quantum effects in the recombination reaction given above are responsible for the famous mystery -- the anomalous isotope effect in ozone formation. This effect still remains poorly understood, mainly due to computational difficulties associated with quantum dynamics treatment of polyatomic systems. Our mixed quantum-classical approach should help to solve this problem.