1111.3557 (Trond S. Ingebrigtsen et al.)

What constitutes a simple liquid?    [PDF]

Trond S. Ingebrigtsen, Thomas B. Schrøder, Jeppe C. Dyre

Simple liquids are traditionally defined as many-body systems of classical
particles interacting via radially symmetric pair potentials. We suggest that a
simple liquid should be defined instead by the property of having strong
correlation between virial and potential energy equilibrium fluctuations in the
NVT ensemble. There is considerable overlap between the two definitions, but
also some notable differences. For instance, in the new definition simplicity
is not a property of the intermolecular potential only because a liquid is
usually only strongly correlating in part of its phase diagram. Moreover,
according to the new definition not all simple liquids are atomic (i.e., with
radially symmetric pair potentials) and not all atomic liquids are simple. The
main part of the paper motivates the new definition of liquid simplicity by
presenting evidence that a liquid is strongly correlating if and only if its
intermolecular interactions may be ignored beyond the first coordination shell
(FCS). This is demonstrated by NVT simulations of structure and dynamics of 15
atomic and molecular model liquids with a shifted-forces cutoff placed at the
first minimum of the radial distribution function. No proof is given that the
chemical characterization follows from the strong correlation property, but it
is shown to be consistent with the existence of isomorphs in strongly
correlating liquids' phase diagram. Finally, we note that the FCS
characterization of simple liquids calls into question the basis for standard
perturbation theory, according to which the repulsive and attractive forces
play fundamentally different roles for the physics of liquids.

View original: http://arxiv.org/abs/1111.3557