# Negative Interfacial tension

**URL:** https://gromacs.bioexcel.eu/t/negative-interfacial-tension/2784
**Category:** User discussions
**Created:** [September 8, 2021, 11:19pm UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension/2784 "2021-09-08T23:19:52Z")
**Posts on this page:** 3
**Page:** 1

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### Author: ![Raji](https://avatars.discourse-cdn.com/v4/letter/r/eb8c5e/32.png) [@Raji](https://gromacs.bioexcel.eu/u/Raji)
#### Post date: [September 8, 2021, 11:19pm UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension/2784/1 "2021-09-08T23:19:52Z")

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GROMACS version: 2018  
GROMACS modification: No

Hi all, I am simulating an interface model water|lipids|perfluoroCarbons|lipids|water to calculate the interfacial tension of lipid coated perfluorocarbons.

1. For these calculations how you say the system reached equilibration to calculate IFT?
2. Without lipids, system reproduces the experimental interfacial tension (IFT) of PFC in water. After adding lipids, in some cases we observed negative interfacial tension. How to interpret that? (When adding the pegylated lipids, PEG chain primarily situated in the water block and alkyl chain of lipid is at interface)

OPLS force field is used to model the atoms.  
##mdp file  
;Run parameters  
integrator = md ; leap-frog integrator  
nsteps = 5000000 ;  
dt = 0.02 ;

; Output control  
nstxout = 5000 ; save coordinates every 0.2 ps  
nstvout = 5000 ; save velocities every 0.2 ps  
nstenergy = 5000 ; save energies every 0.2 ps  
nstlog = 5000 ; update log file every 0.2 ps

; Bond parameters  
continuation = yes ; first dynamics run  
constraint\_algorithm = lincs ; holonomic constraints  
constraints = all-bonds ; all bonds (even heavy atom-H bonds) constrained  
lincs\_iter = 1 ; accuracy of LINCS  
lincs\_order = 4 ; also related to accuracy

; Neighborsearching  
vdwtype = cut-off  
cutoff-scheme = Verlet  
ns\_type = grid ; search neighboring grid cells  
nstlist = 40 ; 10 fs  
rcoulomb = 1.0 ; short-range electrostatic cutoff (in nm)  
rvdw = 1.0 ; short-range van der Waals cutoff (in nm)

; Electrostatics  
coulombtype = PME ; Particle Mesh Ewald for long-range electrostatics  
pme\_order = 4 ; cubic interpolation  
fourierspacing = 0.16 ; grid spacing for FFT

; Temperature coupling  
tcoupl = vrescale  
tc-grps = PFH\_DSPC\_DSPE\_PEG Water\_and\_ions  
tau\_t = 0.1 0.1  
ref\_t = 298.15 298.15

; Pressure coupling  
pcoupl = Berendsen ; pressure coupling is on for NPT  
pcoupltype = semiisotropic  
tau\_p = 5.0 ; time constant, in ps  
ref\_p = 1.0 1.0  
compressibility = 4.5e-5 4.5e-5  
refcoord\_scaling = com

; Periodic boundary conditions  
pbc = xyz ; 3-D PBC

; Dispersion correction  
DispCorr = EnerPres ; account for cut-off vdW scheme

; Velocity generation  
gen\_vel = no ; velocity generation off after NVT

I would appreciate any suggestions.

Thanks  
Raji

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### Author: ![MichelePellegrino](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/michelepellegrino/32/628_2.png) [@MichelePellegrino](https://gromacs.bioexcel.eu/u/MichelePellegrino)
#### Post date: [September 9, 2021, 7:01am UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension/2784/2 "2021-09-09T07:01:23Z")

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Hi Raji,

are you using poistion restraints to keep lipids in place?

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### Author: ![Raji](https://avatars.discourse-cdn.com/v4/letter/r/eb8c5e/32.png) [@Raji](https://gromacs.bioexcel.eu/u/Raji)
#### Post date: [September 9, 2021, 1:08pm UTC](https://gromacs.bioexcel.eu/t/negative-interfacial-tension/2784/3 "2021-09-09T13:08:00Z")

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Hi

No, i did not. I let them reach the equilibration by calculating the area per lipid. Once the the area per lipid is constant, i run at NVT to calculate IFT.

Thanks  
Raji