g_tcaf(1)

NAME
g_tcaf VERSION 3.2.0

SYNOPSIS
g_tcaf -f traj.trr -s topol.tpr -n index.ndx -ot transcur.xvg -oa tcaf_all.xvg -o tcaf.xvg -of tcaf_fit.xvg -oc tcaf_cub.xvg -ov visc_k.xvg -[no]h -nice int -b time -e time -dt time -[no]w -[no]mol -[no]k34 -wt real

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 k2), 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) k2, 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 k2) 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.

FILES
-f traj.trr Input
 Full precision trajectory: trr trj -s topol.tpr Input, Opt.
 Structure+mass(db): tpr tpb tpa gro g96 pdb xml -n index.ndx Input, Opt.
 Index file -ot transcur.xvg Output, Opt.
 xvgr/xmgr file -oa tcaf_all.xvg Output
 xvgr/xmgr file -o tcaf.xvg Output
 xvgr/xmgr file -of tcaf_fit.xvg Output
 xvgr/xmgr file -oc tcaf_cub.xvg Output, Opt.
 xvgr/xmgr file -ov visc_k.xvg Output
 xvgr/xmgr file

OTHER OPTIONS
-[no]h no
 Print help info and quit -nice int 19
 Set the nicelevel -b time -1
 First frame (ps) to read from trajectory -e time -1
 Last frame (ps) to read from trajectory -dt time -1
 Only use frame when t MOD dt = first time (ps) -[no]w no
 View output xvg, xpm, eps and pdb files -[no]mol no
 Calculate tcaf of molecules -[no]k34 no
 Also use k=(3,0,0) and k=(4,0,0) -wt real 5
 Exponential decay time for the TCAF fit weights