[gmx-users] How to define a vector for gangle command

[delara aghaie]

Dear Gromacs users
My simulation box contains dppc lipid bilayer containing a membrane protein.
I want to calculate the angle between P-N in dppc head group and the bilayer normal (z-axis).
We want to use the gromacs gangle command for this purpose.

The command asks us to define a vector for P-N, while for the axis z it does not require vector definition.
I can make an index group containing all the P and N atoms of the dppc headgroup, but I do not know how to make a vector from these two endpoints to give it to the gangle command for angle calculation.

Your help would be greatly appreciated.
Best regards
Delara