Water is the most important absorber of both long and short range radiation in the atmosphere [1]. Therefore, a proper characterization of all absorption by water vapor is of upmost importance in climate prediction. The spectrum of water monomer has been well studied. In comparison, the role of the water dimer in atmospheric absorption is poorly understood. From a computational view point, simulating this spectrum requires the solution to a 12D problem. However, one may adiabatic separate the monomer and dimer modes [2,3]. This allows the use of rigid 6D monomer approaches which normally involve evaluating a potential energy surface on a specified intermolecular grid with rigid monomers, followed by solution to the nuclear motion problem. We relax the rigid monomer constraint by creating water monomer wavefunctions using DVR3D [4] and then vibrationally average the potential at each intermolecular grid point [2]. We perform the average on the best available water pair potential [5] with a monomer correction term [6] to create a spectroscopically transferable water pair potential [2]. Unfortunately, this leads to 1,306,613,597,184 potential calls which would take around 7000 days on a single CPU to compute. However, the averages at each point on the dimer grid are completely independent from one another: an ideal case for grid computing. We use the UCL Condor grid which is a distributed high-throughput infrastructure of ~1400 Windows PCs using Condor resource management software. Furthermore, storing the potential points would occupy about 10TB of disk/memory, so that the averaging must be done on the fly. We first discuss how these calculations have increased the accuracy of existing VRT calculations in comparison with low temperature experiments [2]. Additionally, we report water dimer VRT levels calculated with excited monomers which allow us to give water dimer spectra in the infrared and visible regions of the atmosphere. The agreement with experimental [7] and lower-level theoretical calculations suggests a small but significant contribution of the water dimer to the water continuum problem in the near IR and visible regions in the atmosphere. This is part of the large consortium, Continuum Absorption at Visible and Infrared wavelengths and its Atmospheric Relevance, or simply CAVIAR [8].1. J. T. Kiehl, K. E. Trenberth, Bull. Am. Meteorol. Soc. 1997, 78, 197-208.2. R. E. A. Kelly, J. Tennyson, G. C. Groenenboom, A. van der Avoird, J. Quant. Spectro. Rad. Trans. 2010, 111, 1262-1276.3. C. Leforestier, R. van Harrevelt, A. van der Avoird, J. Phys. Chem. A 2009, 113 12285�12294.4. J. Tennyson, M. A. Kostin, P. Barletta, G. J. Harris, O. L. Polyansky, J. Ramanlal, N. F. Zobov. Comp. Phys. Comm. 2004, 163, 85-116.5. X. Huang, B. J. Braams, J. M. Bowman, R. E. A. Kelly, J. Tennyson, G. C. Groenenboom, A. van der Avoird. J. Chem. Phys. 2008, 128, 034312.6. S. V. Shirin, N. F. Zobov, R. I. Ovsyannikov, O. L. Polyansky, J. Tennyson. J. Chem. Phys. 2008, 128, 224306.7. D. J. Paynter, I. V. Ptashnik, K. P. Shine, K. M. Shine, R. McPheat, R. G. Williams, Geophys. Res. Lett. 2009, 114, D21301. 8. http://www.met.reading.ac.uk/caviar/

Using a vibrational averaging technique to understand the role of water dimers in atmospheric absorption

240th National Meeting (2010)

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