GMX-TCAF(1)

✨ gmx-tcaf — calculate viscosities of liquids ✨

okay bestie, welcome to the GROMACS corner of the manual 💖 this one lives in the gmx-tcaf(1) page and it is literally about figuring out how thick and syrupy your liquid is. let's get into it 🫶

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🌸 NAME 🌸

gmx-tcaf — Calculate viscosities of liquids

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🎀 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>]

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💅 DESCRIPTION 💅

so gmx tcaf computes tranverse current autocorrelations (yes that's how it's spelled in the source, we're keeping it fr fr 🙈). what are those for? estimating the shear viscosity, eta. for the full lore see: Palmer, Phys. Rev. E 49 (1994) pp 359-366. 📚

k so here's how the k-vectors shake out, and bestie this part is very specific:

• (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 (heads up, these vectors are not independent)
• (1,1,1) and the 3 other box diagonals (also not independent)

for every single k-vector it uses the sine and cosine, combined with the velocity in 2 perpendicular directions. do the math and you get 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 gets 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 hands you 16 values of tau and eta. the fit weights decay exponentially with time constant w (that's the one you set with -wt) as exp(-t/w), and the TCAF plus its fit are calculated up to time 5*w. then those eta values should be fitted to 1 - a eta(k) k^2, and from that you can estimate the shear viscosity at k=0. iconic behavior honestly 👑

if the box is cubic, you 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. don't mix those up bestie 😭

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. infinite. wavelength. slay. ✨

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. like, do not skip this. the fit will be giving nothing if you skimp on frames 💀

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🌈 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

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🦋 SEE ALSO 🦋

• gmx(1)

More information about GROMACS is available at <http://www.gromacs.org/>.

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👑 COPYRIGHT 👑

2026, GROMACS development team

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and that's the whole page bestie, no cap 🫶 this was GROMACS version 2026.4, dated Oct 02, 2026, and it's the GMX-TCAF(1) manual page. go get those viscosity numbers, you're the main character of this simulation 💖✨🎀