[gmx-users] no groups in indexfile!

[Amir Abbasi]

Hi !
I want to use g_sgangle to find distance between two groups (each group includes 3 atoms).
I use this command:
g_sgangle -s md_1.tpr -f md_1_noPBC.xtc -n index.ndx -oa 12_angle.xvg -od 12_dist.xvg
but i got this error
Fatal error:
Error: no groups in indexfile

my index file:
 [ G3 ]
76    81    85
 [ G4 ]
110    115    119

Best Regards,
Amir