[gmx-users] No group in index file?

[Xu Dong Huang]

Dear all.

Since I am interested in finding the average angle, dihedrals in my system, I attempted to use g_angle, but then I realized I need to make index file using mk_angndx,

I issued the following:
mk_angndx -s npt.tpr -n angle.ndx

and then i used g_angle -f not.xtc -n angle.ndx -ov angle.xvg -noperiodic -type angle 

and then I get the following error: 
Fatal error:
Error: no groups in indexfile

How can my index file not have the groups in there? How can I go about fixing this

Any insight can be appreciated

Thanks, 

Xu Huang