8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P8.5What is the best way to identify the underlying inversepower-law exponent in strongly correlating liquids?Nicholas Bailey, 1 Thomas Schrøder, 1 and Jeppe Dyre 11 Roskilde University, Center for Glass and Time, Dept. of Sciences, Universitetsvej 1,Box 260 4000, Roskilde, DenmarkRecent work [1, 2] has demonstrated that a class of model liquids, typically those with Lennard-Jones-like (LJ) pair interactions, called strongly correlating liquids, exhibit certain scalingproperties normally associated with systems which interact via inverse power-law (IPL) potentials.These include strong correlation of instantaneous potential energy and virial, and invariance ofstructure, dynamics, and several thermodynamic quantities along certain curves—isomorphs—inthe density-temperature plane [2]. The explanation can be traced to the fact that the LJ potentialis well approximated by an IPL plus a linear term which fluctuates little at fixed volume andtherefore has little effect on structure or dynamics[1]. Thus it makes sense to look at the LJ systemin terms of a IPL reference system plus a perturbation. In fact recently an IPL reference systemwas shown to reproduce both structure and dynamics of a supercooled binary LJ liquid [3], whichwas not the case when using a WCA reference system [4]. It is of interest to identify the exponentof the underlying IPL. One can be identified in several ways, which in general yield differentvalues. In this work we use a variational method which optimizes the estimate of the liquid’sfree energy to identify the exponent for single-component and binary LJ systems. This yieldsIPL reference systems which best match the structure and dynamics of the real LJ systems, butthe variation is relatively small, so that actually a range of exponents can match the real systemalmost equally well, provided the prefactor is chosen optimally. This implies the existence of(near) isomorphs between different IPL systems.[1] N. P. Bailey et al. , J. Chem. Phys. 129, 184507 and 184508 (2008).[2] N. Gnan et al. , J. Chem. Phys. 131, 234504 (2009).[3] U. R. Pedersen et al, Phys. Rev. Lett. , 105, 157801 (2010).[4] L. Berthier and G. Tarjus, Phys. Rev. Lett. 103, 170601 (2009)5