g_rdf_d (1) - Linux Manuals

g_rdf_d: calculates radial distribution functions


g_rdf - calculates radial distribution functions



g_rdf -f traj.xtc -s topol.tpr -n index.ndx -o rdf.xvg -sq sq.xvg -cn rdf_cn.xvg -hq hq.xvg -[no]h -nice int -b time -e time -dt time -[no]w -[no]xvgr -bin real -[no]com -rdf enum -[no]pbc -[no]norm -[no]xy -cut real -ng int -fade real -nlevel int -startq real -endq real -energy real


The structure of liquids can be studied by either neutron or X-ray scattering. The most common way to describe liquid structure is by a radial distribution function. However, this is not easy to obtain from a scattering experiment.

g_rdf calculates radial distribution functions in different ways. The normal method is around a (set of) particle(s), the other method is around the center of mass of a set of particles. With both methods rdf's can also be calculated around axes parallel to the z-axis with option -xy.

The option -rdf sets the type of rdf to be computed. Default is for atoms or particles, but one can also select center of mass or geometry of molecules or residues. In all cases only the atoms in the index groups are taken into account. For molecules and/or the center of mass option a run input file is required. Other weighting than COM or COG can currently only be achieved by providing a run input file with different masses. Option -com also works in conjunction with -rdf.

If a run input file is supplied ( -s) and -rdf is set to atom, exclusions defined in that file are taken into account when calculating the rdf. The option -cut is meant as an alternative way to avoid intramolecular peaks in the rdf plot. It is however better to supply a run input file with a higher number of exclusions. For eg. benzene a topology with nrexcl set to 5 would eliminate all intramolecular contributions to the rdf. Note that all atoms in the selected groups are used, also the ones that don't have Lennard-Jones interactions.

Option -cn produces the cumulative number rdf, i.e. the average number of particles within a distance r.

To bridge the gap between theory and experiment structure factors can be computed (option -sq). The algorithm uses FFT, the gridspacing of which is determined by option -grid.


-f traj.xtc Input
 Trajectory: xtc trr trj gro g96 pdb cpt 

-s topol.tpr Input, Opt.
 Structure+mass(db): tpr tpb tpa gro g96 pdb 

-n index.ndx Input, Opt.
 Index file 

-o rdf.xvg Output, Opt.
 xvgr/xmgr file 

-sq sq.xvg Output, Opt.
 xvgr/xmgr file 

-cn rdf_cn.xvg Output, Opt.
 xvgr/xmgr file 

-hq hq.xvg Output, Opt.
 xvgr/xmgr file 


 Print help 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)

 View output xvg, xpm, eps and pdb files

 Add specific codes (legends etc.) in the output xvg files for the xmgrace program

-bin real 0.002
 Binwidth (nm)

 RDF with respect to the center of mass of first group

-rdf enum atom
 RDF type:  atom mol_com mol_cog res_com or  res_cog

 Use periodic boundary conditions for computing distances. Without PBC the maximum range will be three times the larges box edge.

 Normalize for volume and density

 Use only the x and y components of the distance

-cut real 0
 Shortest distance (nm) to be considered

-ng int 1
 Number of secondary groups to compute RDFs around a central group

-fade real 0
 From this distance onwards the RDF is tranformed by g'(r) [g(r)-1] exp(-(r/fade-1)2 to make it go to 1 smoothly. If fade is 0.0 nothing is done.

-nlevel int 20
 Number of different colors in the diffraction image

-startq real 0
 Starting q (1/nm) 

-endq real 60
 Ending q (1/nm)

-energy real 12
 Energy of the incoming X-ray (keV) 



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