[gmx-users] Surface Tension of a lipid bilayer

[toma0052]

Hello,
     I have a question about calculating the surface tension for the
water-lipid interface of a lipid bilayer.  After an MD run, I can use my .edr
file and g_energy to calculate the surface tension.  I believe that the
surface tension is calculated something like gamma = (Pzz-(Pxx+Pyy)/2)/L.  I
was wondering though, is the pressure tensor and hence surface tension
calculated over the entire simulation box?  I would assume so, since I did
not specify any molecules.  To find the water-lipid surface tension, would I
not need to sum over only the molecules at the surface?  Would this be
do-able in Gromacs?  Would I have to write something separate to do this?
     In my coordinate file, I have DPPC atoms, solvent atoms, and other atoms
(those that make up the head group ie O,N etc.).  Could I just make an index
group that is not DPPC and not Solvent, and sum over those atoms?

Thanks,
Mike Tomasini