[gmx-users] angle distribution between a vector and Z axis

[Marzieh Saeedi Masineh]

Dears, 

I simulated a DMPC lipid bilayer. I want to calculate the distribution of angle between phosphorus - nitrogen vector (P-N vector) and Z axis. Is there any command to calculate angle distribution between a vector and an axis? My Gromacs version is 4.5.4. 

I searched and found gangle command for Gromacs version 5 that can calculate this angle. How can I calculate this with 4.5.4 version?