g_tcaf Main Table of Contents

VERSION 3.1 Thu 28 Feb 2002

Description

g_tcaf computes tranverse current autocorrelations. These are used to estimate the shear viscosity eta. For details see: Palmer, JCP 49 (1994) pp 359-366.

Transverse currents are calculated using the k-vectors (1,0,0) and (2,0,0) each also in the y- and z-direction, (1,1,0) and (1,-1,0) each also in the 2 other plains (these vectors are not independent) and (1,1,1) and the 3 other box diagonals (also not independent). For each k-vector the sine and cosine are used, in combination with the velocity in 2 perpendicular directions. This gives a total of 16*2*2=64 transverse currents. One autocorrelation is calculated fitted for each k-vector, which gives 16 tcaf's. Each of these tcaf's is fitted to f(t) = exp(-v)(cosh(Wv) + 1/W sinh(Wv)), v = -t/(2 tau), W = sqrt(1 - 4 tau eta/rho k^2), which gives 16 tau's and eta's. The fit weights decay with time as exp(-t/wt), the tcaf and fit are calculated up to time 5*wt. The eta's should be fitted to 1 - a eta(k) k^2, from which one can estimate the shear viscosity at k=0.

When the box is cubic, one can use the option -oc, which averages the tcaf's over all k-vectors with the same length. This results in more accurate tcaf's. Both the cubic tcaf's and fits are written to -oc The cubic eta estimates are also written to -ov.

With option -mol the transverse current is determined of molecules instead of atoms. In this case the index group should consist of molecule numbers instead of atom numbers.

The k-dependent viscosities in the -ov file should be fitted to eta(k) = eta0 (1 - a k^2) to obtain the viscosity at infinite wavelength.

NOTE: make sure you write coordinates and velocities often enough. The initial, non-exponential, part of the autocorrelation function is very important for obtaining a good fit.

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