Guest Commentary pubs.acs.org/JPCL
Simulation and Experiment A Difficult Interaction
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ndercooled liquids and the glass transition, even after many decades of active research, still exert the fascination of an unsolved problem holding the tantalizing promise of a simple solution. The interest is further stimulated by the development of beautiful new experimental techniques. Two of these, dielectric measurements under pressure allowing study of the same relaxation at different volumes and temperatures1 and molecular beam deposition on a cold surface, able to study the thermodynamics of an otherwise inaccessible undercooled liquid,2 have been described in two of the accompanying Perspectives. In the old times, progress proceeded by the combination of new experimental techniques and new theoretical ideas. In our time, the situation has changed; one now has to include new numerical results, which in principle belong to the new experimental techniques. However, the computer simulators recruit from the theoreticians because the numerical techniques have been developed in theoretical institutes. Therefore, they usually share the theoreticians’ weakness: it is hard for them to appreciate the significance of new experimental results because they are unable to judge experimental problems. As a consequence, they usually consider that they have done their duty if they reference to all other numerical work in the field (though one has to admit that there begin to be positive exceptions from this rule3). On the other hand, one cannot deny that an increasing part of our knowledge stems from numerical simulation. The brilliant and very detailed review by Cavagna4 demonstrates this very convincingly. The fact that this review is referenced at the beginning of the Perspective of Smith and Kay2 shows that at least some experimentalists are aware of this numerical progress. However, though Cavagna gives a beautiful review of the connections between the numerical evidence, the mode coupling theory,5 the spin glass theory,6 and the random firstorder theory,7 he does not say a word about the secondary relaxations that are so easily seen in a dielectric experiment.1 This is understandable because they are not as easily seen in a numerical simulation. The time steps of a molecular dynamics calculation are on the order of femtoseconds because they have to be short compared to the period of the vibrations. Therefore, it is difficult to reach the separation of a secondary relaxation from the primary one in the microsecond range on the computer. Personally, I would like to see a more intense interaction between these two different ways of exploring the glass transition, the experimental and the numerical one. My guess is that the field could profit a lot.
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REFERENCES
(1) Capaccioli, S.; Paluch, M.; Prevosto, D.; Wang, L.-M.; Ngai, K. L. Many-Body Nature of Relaxation Processes in Glass-Forming Systems. J. Phys. Chem. Lett. 2012, 6, 735−743. (2) Smith, R.; Kay, B. Breaking Through the Glass Ceiling: Recent Experimental Approaches to Probe the Properties of Supercooled Liquids near the Glass Transition. J. Phys. Chem. Lett. 2012, 6, 725− 730. (3) Pedersen, U. R.; Gnan, N.; Bailey, N. P.; Schroeder, T. B.; Dyre, J. C. Strongly Correlating Liquids and Their Isomorphs . J. Non-Cryst. Solids 2011, 357, 320. (4) Cavagna, A. Supercooled Liquids for Pedestrians. Phys. Rep. 2009, 476, 51. (5) Götze, W.; Sjögren, S. Relaxation Processes in Supercooled Liquids. Rep. Prog. Phys. 1992, 55, 241. (6) Mezard, M.; Parisi, G.; Virasoro, M. A. Spin-Glass Theory and Beyond; World Scientific, Singapore, 1987. (7) Kirkpatrick, T. R.; Thirumalai, D.; Wolynes, P. G. Scaling Concepts for the Dynamics of Viscous Liquids near an Ideal Glassy State. Phys. Rev. A 1989, 40, 1045.
U. Buchenau*
Jülich Center of Neutron Science, Forschungszentrum Jülich Postfach 1913, D-52425 Jülich, Federal Republic of Germany Received: February 14, 2012 Accepted: February 23, 2012 Published: March 15, 2012 © 2012 American Chemical Society
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dx.doi.org/10.1021/jz300194h | J. Phys. Chem. Lett. 2012, 3, 760−760
