[ESPResSo] Surface Tension - simplebilayer.tcl

[Salvador Herrera Velarde]

Dear All,
 I need to calculate the surface tension of a planar membrane.
 I believe that the surface tension is calculated something like:
 gamma = Lz*(Pzz-(Pxx+Pyy)/2).
 So, if this equation is correct, I have to calculate the pressure tensor and
 then using the components (average) Pzz,Pxx & Pyy to obtain gamma, right?
 I was wondering though, is the pressure tensor and hence surface tension
 calculated over the entire simulation box? Should I divide by two?
 
 Besides, I have been working a little with mbtools package. I ran some simulations
 for the simplebilayer script, but I get different results with the analytical and
 tabulated potentials.
 
 Here are some lines of the scripts:
 ====================================================================================================
 set moltypes [list { 0 lipid { 0 1 1 } { 0 1 } } ]
 
 ===Tabulated
 lappend nb_interactions [list 0 0 tabulated 9_095_11.tab ]
 lappend nb_interactions [list 0 1 tabulated 9_095_11.tab ]
 lappend nb_interactions [list 1 1 tabulated n9_c160_22.tab ]
 
 ===Defined Potential
 set lj_eps 1.0
 set lj_cutoff 2.5
 set ljshift 0.0
 set ljoffset 0.0
 set lj_sigmah 0.95
 set lj_sigma 1.0
 
 lappend nb_interactions [list 0 0 lennard-jones $lj_eps $lj_sigmah [expr 1.1225*$lj_sigmah] [expr 0.25*$lj_eps] $ljoffset ]
 lappend nb_interactions [list 0 1 lennard-jones $lj_eps $lj_sigmah [expr 1.1225*$lj_sigmah] [expr 0.25*$lj_eps] $ljoffset ]
 lappend nb_interactions [list 1 1 lennard-jones $lj_eps $lj_sigma  [expr 1.1225*$lj_sigma] [expr 0.25*$lj_eps] $ljoffset ]
 
 # We use a special type of cosine attractive potential for the tail tail interactions.
 lappend nb_interactions [list 1 1 lj-cos2 $lj_eps $lj_sigma $ljoffset 1.6 ]
 ===========================================================================================================
 I checked my scripts but I couldn't find the mistake.
 Could you help me with this issue?
 
 Thanks a lot for your time.
 
 Salvador