1 .TH g_rdf 1 "Thu 16 Oct 2008"
3 g_rdf - calculates radial distribution functions
10 .BI "-n" " index.ndx "
13 .BI "-cn" " rdf_cn.xvg "
32 .BI "-startq" " real "
34 .BI "-energy" " real "
36 The structure of liquids can be studied by either neutron or X-ray
37 scattering. The most common way to describe liquid structure is by a
38 radial distribution function. However, this is not easy to obtain from
39 a scattering experiment.
42 g_rdf calculates radial distribution functions in different ways.
43 The normal method is around a (set of) particle(s), the other method
44 is around the center of mass of a set of particles.
45 With both methods rdf's can also be calculated around axes parallel
46 to the z-axis with option
53 sets the type of rdf to be computed.
54 Default is for atoms or particles, but one can also select center
55 of mass or geometry of molecules or residues. In all cases only
56 the atoms in the index groups are taken into account.
57 For molecules and/or the center of mass option a run input file
59 Other weighting than COM or COG can currently only be achieved
60 by providing a run input file with different masses.
63 also works in conjunction with
67 If a run input file is supplied (
75 in that file are taken into account when calculating the rdf.
78 is meant as an alternative way to avoid
79 intramolecular peaks in the rdf plot.
80 It is however better to supply a run input file with a higher number of
81 exclusions. For eg. benzene a topology with nrexcl set to 5
82 would eliminate all intramolecular contributions to the rdf.
83 Note that all atoms in the selected groups are used, also the ones
84 that don't have Lennard-Jones interactions.
89 produces the cumulative number rdf,
90 i.e. the average number of particles within a distance r.
93 To bridge the gap between theory and experiment structure factors can
96 ). The algorithm uses FFT, the gridspacing of which is determined by option
102 Trajectory: xtc trr trj gro g96 pdb cpt
104 .BI "-s" " topol.tpr"
106 Structure+mass(db): tpr tpb tpa gro g96 pdb
108 .BI "-n" " index.ndx"
120 .BI "-cn" " rdf_cn.xvg"
130 Print help info and quit
132 .BI "-nice" " int" " 19"
135 .BI "-b" " time" " 0 "
136 First frame (ps) to read from trajectory
138 .BI "-e" " time" " 0 "
139 Last frame (ps) to read from trajectory
141 .BI "-dt" " time" " 0 "
142 Only use frame when t MOD dt = first time (ps)
145 View output xvg, xpm, eps and pdb files
147 .BI "-[no]xvgr" "yes "
148 Add specific codes (legends etc.) in the output xvg files for the xmgrace program
150 .BI "-bin" " real" " 0.002 "
154 RDF with respect to the center of mass of first group
156 .BI "-rdf" " enum" " atom"
169 .BI "-[no]pbc" "yes "
170 Use periodic boundary conditions for computing distances. Without PBC the maximum range will be three times the larges box edge.
172 .BI "-[no]norm" "yes "
173 Normalize for volume and density
176 Use only the x and y components of the distance
178 .BI "-cut" " real" " 0 "
179 Shortest distance (nm) to be considered
181 .BI "-ng" " int" " 1"
182 Number of secondary groups to compute RDFs around a central group
184 .BI "-fade" " real" " 0 "
185 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.
187 .BI "-nlevel" " int" " 20"
188 Number of different colors in the diffraction image
190 .BI "-startq" " real" " 0 "
193 .BI "-endq" " real" " 60 "
196 .BI "-energy" " real" " 12 "
197 Energy of the incoming X-ray (keV)