[gmx-users] RDF of a group around CNT axis

[Sajjad Kavyani]

Dear experts,
I want to calculate rdf of a certain group around the axis of a cnt, but I
do not know what are the proper parameters to choose in the command-line.
To test the parameters, I calculated the rdf of cnt itself around its axis
which must be a sharp peak at the cnt radius.
I tested the following: (both selection groups of reference and rdf are CNT)
gmx rdf -f md.gro -s md.tpr  -o trdf.xvg -xy -selrpos mol_com
gmx rdf -f md.gro -s md.tpr  -o trdf.xvg -xy -selrpos res_com
gmx rdf -f md.gro -s md.tpr  -o trdf.xvg -xy -selrpos whole_res_com
gmx rdf -f md.gro -s md.tpr  -o trdf.xvg -xy -selrpos whole_mol_com

BUT all the results are the same!!! and surprisingly they have multiple
sharp peaks at different ranges, which I expected just one peak at the
radius of the CNT !!!!
It is notable that the CNT is aligned to z axis.
(the first two peaks are listed below):

      0.000 211939.438
      0.002 53771.996
      0.004 11278.401
      0.006 3935.633
      0.008 1782.280
      0.010  809.616
      0.012  584.723
      0.014  536.531
      0.016  292.361
      0.018  149.929
      0.020   94.455
      0.022   00.000
          ...
          ...
          ...
      0.118    00.000
      0.120    0.375
      0.122    2.212
      0.124    0.725
      0.126    6.069
      0.128   36.545
      0.130  114.868
      0.132  278.390
      0.134 1669.916
      0.136 2360.069
      0.138  346.139
      0.140  183.770
      0.142   39.277

Could you please explain to me that what should I do?