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.

- For these calculations how you say the system reached equilibration to calculate IFT?
- 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