[gmx-users] Trouble with gmx distance

[Sotirios Dionysios I. Papadatos]

Hi, I am trying to calculate the distance between two atoms, which are defined in an index file. The command that I use is:

"gmx distance -f output.gro -s prod.tpr -n index.ndx -oav distance.xvg"

I get the obvious:

"Selection 'atom_name' does not evaluate into an even number of positions (there

are 1 positions)"
The same happens if I use in -f prod.trr
What might be wrong? Any insights on what to look for? What usually gives this kind of error?
Feel free to ask for any further information.
Thanks in advance