 ## gmx tcaf

### Synopsis

```gmx tcaf [-f [<.trr/.cpt/...>]] [-s [<.tpr/.tpb/...>]] [-n [<.ndx>]]
[-ot [<.xvg>]] [-oa [<.xvg>]] [-o [<.xvg>]] [-of [<.xvg>]]
[-oc [<.xvg>]] [-ov [<.xvg>]] [-nice ] [-b ] [-e ]
[-dt ] [-[no]w] [-xvg ] [-[no]mol] [-[no]k34]
[-wt ] [-acflen ] [-[no]normalize] [-P ]
[-fitfn ] [-beginfit ] [-endfit ]
```

### Description

gmx tcaf computes tranverse current autocorrelations. These are used to estimate the shear viscosity, η. 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 τ), W = sqrt(1 - 4 τ η/ρ k^2), which gives 16 values of τ and η. 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 η values should be fitted to 1 - a η(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 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 η 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 η(k) = η_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 important for obtaining a good fit.

### Options

Options to specify input and output files:

-f [<.trr/.cpt/...>] (traj.trr) (Input)
Full precision trajectory: trr cpt trj tng
-s [<.tpr/.tpb/...>] (topol.tpr) (Input, Optional)
Structure+mass(db): tpr tpb tpa gro g96 pdb brk ent
-n [<.ndx>] (index.ndx) (Input, Optional)
Index file
-ot [<.xvg>] (transcur.xvg) (Output, Optional)
xvgr/xmgr file
-oa [<.xvg>] (tcaf_all.xvg) (Output)
xvgr/xmgr file
-o [<.xvg>] (tcaf.xvg) (Output)
xvgr/xmgr file
-of [<.xvg>] (tcaf_fit.xvg) (Output)
xvgr/xmgr file
-oc [<.xvg>] (tcaf_cub.xvg) (Output, Optional)
xvgr/xmgr file
-ov [<.xvg>] (visc_k.xvg) (Output)
xvgr/xmgr file
Other options:

-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]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, vac, exp5, exp7, exp9, erffit
-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

http://www.gromacs.org