[gmx-users] Surface Tension of a lipid bilayer

[Eric Jakobsson]

The theory underlying this is reviewed in our group's 1995 
Biophysical Journal (Chiu et al) paper where we introduced the 
surface tension boundary conditions---a deeper look at the theory is 
in in Steve White's earlier paper in PNAS that is referenced in our 
'95 paper.  The short answer is yes, you include all the atoms in the 
system.  That reflects the fact that the surface tension, as measured 
in a laboratory, is a property of the entire system.  You can also 
slice the system up and see what components contribute how much to 
the surface tension, but that is a separate (although related) issue.

At 04:59 PM 12/1/2006, you wrote:
>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
>
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---------------------------------
Eric Jakobsson, Ph.D.
Professor, Department of Molecular and Integrative Physiology, and of 
Biochemistry, and of the Center for Biophysics and Computational Biology
Senior Research Scientist, National Center for Supercomputing Applications
Professor, Beckman Institute for Advanced Science and Technology
3261 Beckman Institute, mc251
University of Illinois, Urbana, IL 61801
ph. 217-244-2896  Fax 217 244 9757