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Broad audience summary of

A repulsive reference potential reproducing the dynamics of a liquid with attractions
Ulf R. Pedersen, Thomas B. Schrøder, Jeppe C. Dyre
[Phys. Rev. Lett. 105, 157801] [PDF]

Why some liquids are simple

Liquids condense due to long-range attractive forces between molecules, yet short-range repulsive forces make them hard to compress. In this paper we demonstrate that, nevertheless, only repulsive forces determine flow dynamics - even at low temperatures where flow is sluggish and molecules have long-range collective motions.

This is a somewhat surprising finding. Just last year Berthier and Tarjus came to the opposite conclusion using a conventional theory to split interactions into repulsive and attractive parts. However, in the novel approach, repulsive forces decay quickly with distance in the form of inverse power laws. This makes repulsive interactions scale invariant, that is that they look the same from any distance. A consequence is simpler physics. For example, this explains recent experimental findings that for some liquids cooling and compression have the same effect on flow dynamics.

As indicated in the title, it should be noted that some liquids indeed are more complicated due to strong short-range attractive forces. An infamous example is water, know for its many peculiar properties. Van der Waals liquids (e.g. many organic liquids) and perhaps metals are believed to inherit the simplicity of repulsive inverse power-law interactions.