Hello Alevilla,
Thanks for your reply. I have read the syntax and able to get the results but data doesn’t seem right. I have used following command:
gmx_mpi distance -f file.xtc -s file.tpr -n cog.ndx -oav distancecentroid.xvg
cog.ndx:
[A]
854 852 850 849 857 859
[B]
981 979 977 976 984 986
where A is ring 1 and B is ring 2. I made the file manually.