**Compressibility and pressure correlations in isotropic solids and fluids**

JP Wittmer and H Xu and P Polinska and C Gillig and J Helfferich and F Weysser and J Baschnagel, EUROPEAN PHYSICAL JOURNAL E, 36, 1-17 (2013).

DOI: 10.1140/epje/i2013-13131-y

Presenting simple coarse-grained models of isotropic solids and fluids
in d = 1 , 2 and 3 dimensions we investigate the correlations of the
instantaneous pressure and its ideal and excess contributions at either
imposed pressure (NPT-ensemble, lambda = 0 or volume (NVT-ensemble,
lambda = 1 and for more general values of the dimensionless parameter
lambda characterizing the constant-volume constraint. The stress
fluctuation representation of the compression modulus K in the NVT-
ensemble is derived directly (without a microscopic displacement field)
using the well-known thermodynamic transformation rules between
conjugated ensembles. The transform is made manifest by computing the
Rowlinson functional also in the NPT-ensemble where with x = P (id)/K
being a scaling variable, P (id) the ideal pressure and f (0)(x) =
x(2-x) a universal function. By gradually increasing lambda by means of
an external spring potential, the crossover between both classical
ensemble limits is monitored. This demonstrates, e.g., the lever rule
F-Row vertical bar(lambda) - K**lambda + (1 - lambda)f(0)(x)**.

Return to Publications page