Please see my reply on gmx-users.

Hello Abid Ali Channa,

remove -select

Regards

Tiago

Dear Tiago Espinosa de Oliveira,

Thanks for your reply . By removing -select option I am getting this error

" Inconsistency in user input:

Selection 'r_81' does not evaluate into an even number of positions (there are

11 positions"

Dear Abid 

First, make an index file and show those residues  in index file. Check .gro file and write the residue index for example 15ALA (use 15) or 35GLY (use 35). Then, while calculating distance indicate index file: g_dist  .... -n index.ndx  and show those residues. you can get distance between two residues. 

Please see my reply on gmx-users.

Dear Justin Lemkul,

I calculated easily with the help of your reply.

Thanks for all suggestions.

Dear Abid,

Could you please tell me how you solved this? I am facing same errors.

Hello Souvik ,

Even I was also facing the same problem from one day but today I found that I was giving the wrong selection command in index file . Suppose you want to calculate distance between two residues of their CA atom , then you give "r1|r2| & a CA " in index file group and then give the command gmx distance -f md.xtc -s md.tpr -n index.ndx -oav distance.xvg . Because each selection should consist of pairs of positions . In such types of problems normally we give wrong selection in index file and that we can rectify it by help command .

Dear Surabhi,

Thank you so much for your clarification.