NAME

       g_velacc - calculates velocity autocorrelation functions

       VERSION 4.6.5

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/>.

                                          Mon 2 Dec 2013                              g_velacc(1)