Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

Differential Cross Section (Å 2 sr -1 )
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Scattering Angle (deg)
Figure 3.6: Computed and measured DCS of water at 6 eV. The present elastic (rotationally summed) DCS
is given by the thick solid line. Other lines denote partial state-to-state DCS (dashed line: 0 → 1; solid line:
0 → 0). The filled squares correspond to the experimental elastic DCS of (Cho et al. 2004 J. Phys. B, 37
4639). The open circles and diamonds correspond respectively to the experimental pure elastic (∆j = 0) and
rotationally inelastic (∆j = ±1) DCS of (Jung et al. 1982 J. Phys. B, 15 3535); the sum of both contributions
is given by the stars. Taken from Gorfinkiel et al. (2005 Eur. Phys. J. D, 35, 231).
3.4 High Performance Computing
The Astromol team has been involved in several high performance computing initiatives to achieve its objectives.
The most demanding issue was the construction of multi-dimensional ab-initio potential energy surfaces and their
calibration to cm −1 accuracy using our explicitely correlated coupled cluster approach. The inelastic scattering
calculations proved also very demanding, both at the classical level (to investigate the H2O quenching) and at
the close coupling level (CO colliding with H2 and HC3N colliding with He and H2). In the future, exploration
of multi-parametric models of astrophysical sources may also prove very demanding, as was already explored
by S. Maret.
In order to achieve our goals at optimal cost, we combined a hierarchy of approaches and machines.
• Our development benefited of a few dedicated PCs acquired with support of PCMI and CNES, and
complemented at the end of 2004 by a 64-bit opteron biprocessor with 8 GB memory which was devoted
to explicitely correlated investigations. This latter machine was also used as a testbed for the preparation
of the invitation to tender for the upgrade of the computing center of the Observatory (see below).
• Large ab-initio production calculations, either conventional or explicitely correlated, were run on the
national supercomputers. We submitted a huge demand in 2005 to boost the calculations of all the
potential energy surfaces related to the “Molecular Universe” FP6 project. We obtained 60000 hours on
the IDRIS and the CEA supercomputers and 100000 hours on the CINES.
• Multi-dimensional potential energy surfaces were sampled by a multi-parametric Monte Carlo procedure.
The 9-D surface for H2O-H2 required 375000 independent geometries, each combining a counterpoise of
3 ab-initio calculations resulting into over a million of short ab-initio runs. This computational challenge
triggered a pluri-disciplinary development of the Grenoble computer grid “CiGri” with the support of
a 3-year CDD funded by the ACI GRID and a strong involvment of the STIC community (INRIA and
IMAG). A CiGrid protopype permitted to gather about 200000 cpu hours on idle PCs and to achieve the
9-D surface for H2O-H2 within two months. Presently CiGri matured to a convivial middleware, and has
been used for classical scattering calculations in our team and by other projects (including optimisation
studies for the MARSIS sub-surface radar observations on Mars Express).
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