Berlin 2005 – scientific programme

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DY: Dynamik und Statistische Physik

DY 14: Complex Fluids

DY 14.12: Talk

Friday, March 4, 2005, 18:15–18:30, TU H3010

Why are effective potentials soft? — •Sabine H.L. Klapp — Stranski-Laboratorium für Physikalische und Theoretische Chemie, Sekr. TC7, Technische Universität Berlin, Strasse des 17. Juni 124, 10623 Berlin

In this contribution we focus on the question why effective potentials W(R) between interacting supramolecular are typically “soft”, i.e., remain finite even for R→ 0 and vary much slower with R than the underlying interatomic interaction potentials. To this end we consider a number of special model systems, which can be treated semi–analytically, starting with an atom and a diatomic. However, the characteristic “softness” of mesoscopic effective potentials is recovered only for our most complex model, that is two “glassy disks” with liquid–like, yet frozen internal configurations of atoms. In this case W(R) varies so slowly that it can be parametrized by estimating the free energy change associated with the disk’s overlap. The resulting overlap approximation behaves qualitatively like ad hoc effective potentials used in dissipative particle dynamics (DPD) simulations [1]. We also show that W(R) vanishes when the molecular units to be coarse–grained are non–bonded [2]. This sheds some doubts on the widely used procedure of coarse–graining water molecules in DPD simulations.

1) S. H. L. Klapp, D. J. Diestler, and M. Schoen, J. Phys.: Condens. Matter 16, 7331 (2004). 2) H. Bock, K. E. Gubbins, and S. H. L. Klapp, in preparation.