.TH gmx-rotacf 1 "" "VERSION 5.0.2" "GROMACS Manual"
.SH NAME
gmx-rotacf \- Calculate the rotational correlation function for molecules

.SH SYNOPSIS
gmx rotacf [\-f [<.xtc/.trr/...>]] [\-s [<.tpr/.tpb/...>]]
           [\-n [<.ndx>]] [\-o [<.xvg>]] [\-nice <int>] [\-b <time>]
           [\-e <time>] [\-dt <time>] [\-[no]w] [\-xvg <enum>] [\-[no]d]
           [\-[no]aver] [\-acflen <int>] [\-[no]normalize] [\-P <enum>]
           [\-fitfn <enum>] [\-beginfit <real>] [\-endfit <real>]

.SH DESCRIPTION
\fBgmx rotacf\fR calculates the rotational correlation function for molecules. Atom triplets (i,j,k) must be given in the index file, defining two vectors ij and jk. The rotational ACF is calculated as the autocorrelation function of the vector n = ij x jk, i.e. the cross product of the two vectors. Since three atoms span a plane, the order of the three atoms does not matter. Optionally, by invoking the \fB\-d\fR switch, you can calculate the rotational correlation function for linear molecules by specifying atom pairs (i,j) in the index file.

EXAMPLES

\fBgmx rotacf \-P 1 \-nparm 2 \-fft \-n index \-o rotacf\-x\-P1 \-fa expfit\-x\-P1 \-beginfit 2.5 \-endfit 20.0\fR

This will calculate the rotational correlation function using a first order Legendre polynomial of the angle of a vector defined by the index file. The correlation function will be fitted from 2.5 ps until 20.0 ps to a two\-parameter exponential.

.SH OPTIONS
Options to specify input and output files:

.BI "\-f" " [<.xtc/.trr/...>] (traj.xtc) (Input)"
    Trajectory: xtc trr cpt trj gro g96 pdb tng

.BI "\-s" " [<.tpr/.tpb/...>] (topol.tpr) (Input)"
    Run input file: tpr tpb tpa

.BI "\-n" " [<.ndx>] (index.ndx) (Input)"
    Index file

.BI "\-o" " [<.xvg>] (rotacf.xvg) (Output)"
    xvgr/xmgr file


Other options:

.BI "\-nice" " <int> (19)"
    Set the nicelevel

.BI "\-b" " <time> (0)"
    First frame (ps) to read from trajectory

.BI "\-e" " <time> (0)"
    Last frame (ps) to read from trajectory

.BI "\-dt" " <time> (0)"
    Only use frame when t MOD dt = first time (ps)

.BI "\-[no]w" "  (no)"
    View output \fB.xvg\fR, \fB.xpm\fR, \fB.eps\fR and \fB.pdb\fR files

.BI "\-xvg" " <enum> (xmgrace)"
    xvg plot formatting: xmgrace, xmgr, none

.BI "\-[no]d" "  (no)"
    Use index doublets (vectors) for correlation function instead of triplets (planes)

.BI "\-[no]aver" "  (yes)"
    Average over molecules

.BI "\-acflen" " <int> (-1)"
    Length of the ACF, default is half the number of frames

.BI "\-[no]normalize" "  (yes)"
    Normalize ACF

.BI "\-P" " <enum> (0)"
    Order of Legendre polynomial for ACF (0 indicates none): 0, 1, 2, 3

.BI "\-fitfn" " <enum> (none)"
    Fit function: none, exp, aexp, exp_exp, vac, exp5, exp7, exp9, erffit

.BI "\-beginfit" " <real> (0)"
    Time where to begin the exponential fit of the correlation function

.BI "\-endfit" " <real> (-1)"
    Time where to end the exponential fit of the correlation function, \-1 is until the end


.SH SEE ALSO
.BR gromacs(7)

More information about \fBGROMACS\fR is available at <\fIhttp://www.gromacs.org/\fR>.