DOKK / manpages / debian 12 / gromacs-data / gmx-velacc.1.en
GMX-VELACC(1) GROMACS GMX-VELACC(1)

gmx-velacc - Calculate velocity autocorrelation functions

gmx velacc [-f [<.trr/.cpt/...>]] [-s [<.tpr/.gro/...>]] [-n [<.ndx>]]


           [-o [<.xvg>]] [-os [<.xvg>]] [-b <time>] [-e <time>]


           [-dt <time>] [-[no]w] [-xvg <enum>] [-[no]m] [-[no]recip]


           [-[no]mol] [-acflen <int>] [-[no]normalize] [-P <enum>]


           [-fitfn <enum>] [-beginfit <real>] [-endfit <real>]


gmx velacc computes the velocity autocorrelation function. When the -m option is used, the momentum autocorrelation function is calculated.

With option -mol the velocity autocorrelation function of molecules is calculated. In this case the index group should consist of molecule numbers instead of atom numbers.

By using option -os you can also extract the estimated (vibrational) power spectrum, which is the Fourier transform of the velocity autocorrelation function. Be sure that your trajectory contains frames with velocity information (i.e. nstvout was set in your original .mdp file), and that the time interval between data collection points is much shorter than the time scale of the autocorrelation.

Options to specify input files:

Full precision trajectory: trr cpt tng
Structure+mass(db): tpr gro g96 pdb brk ent
Index file

Options to specify output files:

xvgr/xmgr file
xvgr/xmgr file

Other options:

Time of first frame to read from trajectory (default unit ps)
Time of last frame to read from trajectory (default unit ps)
Only use frame when t MOD dt = first time (default unit ps)
-[no]w (no)
View output .xvg, .xpm, .eps and .pdb files
xvg plot formatting: xmgrace, xmgr, none
-[no]m (no)
Calculate the momentum autocorrelation function
-[no]recip (yes)
Use cm^-1 on X-axis instead of 1/ps for spectra.
-[no]mol (no)
Calculate the velocity acf of molecules
Length of the ACF, default is half the number of frames
-[no]normalize (yes)
Normalize ACF
Order of Legendre polynomial for ACF (0 indicates none): 0, 1, 2, 3
Fit function: none, exp, aexp, exp_exp, exp5, exp7, exp9
Time where to begin the exponential fit of the correlation function
Time where to end the exponential fit of the correlation function, -1 is until the end

gmx(1)

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2023, GROMACS development team

February 3, 2023 2022.5