g_velacc(1)

NAME
g_velacc - calculates velocity autocorrelation functions VERSION 4.6.2

SYNOPSIS
g_velacc -f traj.trr -s topol.tpr -n index.ndx -o vac.xvg -os spectrum.xvg -[no]h -[no]version -nice int -b time -e time -dt time -[no]w -xvg enum -[no]m -[no]recip -[no]mol -acflen int -[no]normalize -P enum -fitfn enum -ncskip int -beginfit real -endfit real

DESCRIPTION
g_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. 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.

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 -o vac.xvg Output
 xvgr/xmgr file -os spectrum.xvg Output, Opt.
 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]mno
 Calculate the momentum autocorrelation function -[no]recipyes
 Use cm-1 on X-axis instead of 1/ps for spectra. -[no]molno
 Calculate the velocity acf of molecules -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/>.