g_tcaf(1)

NAME
g_tcaf - calculates viscosities of liquids VERSION 4.6.3

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 -[no]version -nice int -b time -e time -dt time -[no]w -xvg enum -[no]mol -[no]k34 -wt real -acflen int -[no]normalize -P enum -fitfn enum -ncskip int -beginfit real -endfit real

DESCRIPTION
g_tcaf computes tranverse current autocorrelations. These are used to estimate the shear viscosity, eta. For details see: Palmer, Phys. Rev. E 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 planes (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 TCAFs. Each of these TCAFs 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 values of tau and eta. The fit weights decay exponentially with time constant w (given with -wt) as exp(-t/w), and the TCAF and fit are calculated up to time 5*w. The eta values 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 TCAFs over all k-vectors with the same length. This results in more accurate TCAFs. Both the cubic TCAFs 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) = eta_0 (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 cpt -s topol.tpr Input, Opt.
 Structure+mass(db): tpr tpb tpa gro g96 pdb -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]hno
 Print help info and quit -[no]versionno
 Print version info and quit -nice int 19
 Set the nicelevel -b time 0
 First frame (ps) to read from trajectory -e time 0
 Last frame (ps) to read from trajectory -dt time 0
 Only use frame when t MOD dt = first time (ps) -[no]wno
 View output .xvg, .xpm, .eps and .pdb files -xvg enum xmgrace
 xvg plot formatting: xmgrace, xmgr or none -[no]molno
 Calculate TCAF of molecules -[no]k34no
 Also use k=(3,0,0) and k=(4,0,0) -wt real 5
 Exponential decay time for the TCAF fit weights -acflen int -1
 Length of the ACF, default is half the number of frames -[no]normalizeyes
 Normalize ACF -P enum 0
 Order of Legendre polynomial for ACF (0 indicates none): 0, 1, 2 or 3 -fitfn enum none
 Fit function: none, exp, aexp, exp_exp, vac, exp5, exp7, exp9 or erffit -ncskip int 0
 Skip this many points in the output file of correlation functions -beginfit real 0
 Time where to begin the exponential fit of the correlation function -endfit real -1
 Time where to end the exponential fit of the correlation function, -1 is until the end

SEE ALSO
gromacs(7) More information about GROMACS is available at <http://www.gromacs.org/>.