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

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