[gmx-users] surface tension II

[Pedro L. Loureiro]

  > On Wed, 6 Feb 2002, paloureiro wrote: 
  > 
  > >is it possible to use the surface tension option with the Parrinello
  algorithm (the manual says it is not) ? 
  > >I have been using the surface-tension with the Berendsen p couple in a DPPC
  bilayer, but my system behaves in a strange way: 
  > >the use of -200 bar.nm or +200 bar.nm leads to a contraction in the XY
  plane (I would expect -200 bar.nm to contract and +200 bar.nm to expand the xy
  area). 
  > >I have used either cut-off and PME electrostatics. I have set the
  compressibility to 9.8e-5 bar-1 in xy and z. 
  > > 
  > >Any suggestion will be very helpful. 
  > I don't know whether something could be wrong with the algorithm, but I 
  > suggest you check the initial conditions: run without P coupling and 
  > measure the pressure (average over > 100 ps). Chances are that your 
  > pressure is very negative already, so the system should contract. Also, 
  > since your compressibility is low you will have only small differences in 
  > area with -200 and +200 bar. Measure (using g_energy) the area and surface 
  > tension after quilibration and then compare again. 
  > 
  > Groeten, David. 
  > _________________________________________________________________ 

  Dear David, 

  once more, thank you for your suggestions! 
  1)my system bears a pressure of 400 atm in the XY plane (the membrane plane)
  in a constant volume simulation (as you suggested) - area per lipid 62.2 A**2.

  2)in another simulation (area per lipid 60 A**2) the pressure fluctuates
  around - 20 atm in the membrane plane. 
  (The smaller the area the greater the intermolecular interactions between the
  headgroups - the greater the virial the smaller the pressure.) 
  3)if I use the semiisotropic option in the .mdp file and set the XY-plane
  pressure to -100 atm (and Z pressure to 1 atm), there is an expansion of the
  membrane (increase in the area per lipid). 
  4)with a value of +100 atm, there is a decrease in the area. 
  5)when I use the surface-tension option (with a value of 200 bar.nm - which is
  equivalent to a mean lateral pressure of -100 atm) the area decreases! 

  Cheers, 
Pedro Loureiro
e-mail: paloureiro at biof.ufrj.br


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