[gmx-users] Help to membranes pressure

Absalom Zamorano absalomz2002 at yahoo.com.mx
Mon Mar 7 14:58:33 CET 2005

Hi everyone, Im trying to apply a pressure of -200 bar
on a mixed of H2O and dppc, to form a bilayer
membrane. So I modified the md.mdp, and I tried to
follow the manual instruction, but results were very
I used:

pcoupltype       = surface_tension
compressibility  = 4.5e-5   0.005
ref_p            = 1809.117  200.0 
//Must I use the signs?

Because...I suposed....
surface_tension: is coupled to the x/y dimensions of
the box.
First value of ref_p is surface_tension*number of
surfaces, and I used 
surface_tension= 9nm (-200bar - (1.013+1.013)bar/2)=
               =9(-201.013) =  -1809.117 
               (from 3.36 eq in the manual, 
                and 1 atm = 1.013bar)
Second value of ref-p is the refence z-pressure [bar]
               = 2000
Two compressibility [bar-1] values are respectives to
x/y and z directions.
    compressibility x/y = 4.5-e5 for  water at 1 atm
and 300 K.
    compresibility z= 1/200bar  =  0.005 bar-1

  I will apprecite very much something of ligth,


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