[gmx-users] Angle between a vector and the normal to a sphere

[ABEL Stephane 175950]

Hi all, 

Is there a command/tool in a gromacs to compute the angle between a vector defined defined two atoms molecule and the normal to a sphere defined by a set of atoms for instance water ? if yes, how 

I use a version of gromacs > 5.0 

Thanks

Stéphane