GMX-TCAF(1) GROMACS GMX-TCAF(1) NAME gmx-tcaf - Calculate viscosities of liquids SYNOPSIS gmx tcaf [-f [<.trr/.cpt/...>]] [-s [<.tpr/.gro/...>]] [-n [<.ndx>]] [-ot [<.xvg>]] [-oa [<.xvg>]] [-o [<.xvg>]] [-of [<.xvg>]] [-oc [<.xvg>]] [-ov [<.xvg>]] [-b <time>] [-e <time>] [-dt <time>] [-[no]w] [-xvg <enum>] [-[no]mol] [-[no]k34] [-wt <real>] [-acflen <int>] [-[no]normalize] [-P <enum>] [-fitfn <enum>] [-beginfit <real>] [-endfit <real>] DESCRIPTION gmx 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 direc- tions. This gives a total of 16*2*2=64 transverse currents. One autocorre- lation 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 k^2), 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) k^2, from which one can es- timate 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 es- timates 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 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 im- portant for obtaining a good fit. OPTIONS Options to specify input files: -f [<.trr/.cpt/...>] (traj.trr) Full precision trajectory: trr cpt tng h5md -s [<.tpr/.gro/...>] (topol.tpr) (Optional) Structure+mass(db): tpr gro g96 pdb brk ent -n [<.ndx>] (index.ndx) (Optional) Index file Options to specify output files: -ot [<.xvg>] (transcur.xvg) (Optional) xvgr/xmgr file -oa [<.xvg>] (tcaf_all.xvg) xvgr/xmgr file -o [<.xvg>] (tcaf.xvg) xvgr/xmgr file -of [<.xvg>] (tcaf_fit.xvg) xvgr/xmgr file -oc [<.xvg>] (tcaf_cub.xvg) (Optional) xvgr/xmgr file -ov [<.xvg>] (visc_k.xvg) xvgr/xmgr file Other options: -b <time> (0) Time of first frame to read from trajectory (default unit ps) -e <time> (0) Time of last frame to read from trajectory (default unit ps) -dt <time> (0) Only use frame when t MOD dt = first time (default unit ps) -[no]w (no) View output .xvg, .xpm, .eps and .pdb files -xvg <enum> (xmgrace) xvg plot formatting: xmgrace, xmgr, none -[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 -acflen <int> (-1) Length of the ACF, default is half the number of frames -[no]normalize (yes) Normalize ACF -P <enum> (0) Order of Legendre polynomial for ACF (0 indicates none): 0, 1, 2, 3 -fitfn <enum> (none) Fit function: none, exp, aexp, exp_exp, exp5, exp7, exp9 -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 gmx(1) More information about GROMACS is available at <http://www.gromacs.org/>. COPYRIGHT 2026, GROMACS development team 2026.3 Jun 25, 2026 GMX-TCAF(1)
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