[gmx-users] gmx distance

[Alex]

v. 5.0.5
I need to calculate the distance between COMs of two groups (both in index
file) as a function of time. I can't think of anything simpler.
 I'm doing:
"gmx distance -f traj.trr -n index.ndx -oav dist.xvg" and then selecting my
groups manually from the menu.

 I am getting:

"Selection 'DG' does not evaluate into an even number of positions (there
are
33 positions)"

To this effect, the manual states something that could have easily come out
of Donald Trump:

"gmx distance calculates distances between pairs of positions as a function
of time. Each selection specifies an independent set of distances to
calculate. Each selection should consist of pairs of positions, and the
distances are computed between positions 1-2, 3-4, etc."

It is my understanding that the distance is calculated not as the distance
between COMs (which is only one pair), but something else. I want the
distance between the COM of my DG and one other group.

Anything I am missing in the supplied options?

Thanks,

Alex