[gmx-users] analysis of angle between two beta sheet of martini protein cg

[SHAHEE ISLAM]

hi,
i want to calculate the angle between the two beeta sheet of a
protein.for this reason i have made a index file containing two beeta
sheet group.can anyone please suggest me how can i do this.
i am using this command
gmx gangle -f ../pbc340.xtc/md-340k-0-1 -s ../equilibration.gro -n
b-1-3-sheet-p1.ndx -oav angle.xvg -dt 10
the error is
Inconsistency in user input:
Number of positions in selection 1 in the first group not divisible by 3

thanking you
shahee