Negative Interfacial tension

User discussions
1

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

2

Hi Raji,

are you using poistion restraints to keep lipids in place?

3

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