Main Table of Contents

Fri 5 Jul 2013


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 methods are around the center of mass of a set of particles (-com) or to the closest particle in a set (-surf). With all methods, the RDF can also be calculated around axes parallel to the z-axis with option -xy. With option -surf normalization can not be used.

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. Weighting other than COM or COG can currently only be achieved by providing a run input file with different masses. Options -com and -surf also work 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 e.g. benzene a topology, setting nrexcl 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 grid spacing 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
-d sfactor.dat Input, Opt. Generic data 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

Other options

-[no]h bool no Print help info and quit
-[no]version bool no 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]w bool no View output .xvg, .xpm, .eps and .pdb files
-xvg enum xmgrace xvg plot formatting: xmgrace, xmgr or none
-bin real 0.002 Binwidth (nm)
-[no]com bool no RDF with respect to the center of mass of first group
-surf enum no RDF with respect to the surface of the first group: no, mol or res
-rdf enum atom RDF type: atom, mol_com, mol_cog, res_com or res_cog
-[no]pbc bool yes Use periodic boundary conditions for computing distances. Without PBC the maximum range will be three times the largest box edge.
-[no]norm bool yes Normalize for volume and density
-[no]xy bool no 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) = 1 + [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)