GROMACS version: 2021.4
GROMACS modification: Yes/No
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I want to measure the IFT between oil and water. At first I ran the simulation with a mixture of hydrocarbons (representing the oil phase) in water at 298 K in an NVT run for 20 ns. Herein, the oil formed a ball surrounded by water. Now, in order to determine the IFT value I have to run an NPAT simulation. I tried using semi-isotropic p-couple type but got negative values for IFT. Should I switch to surface-tension as the pcouple-type? what parameters should I use? I am providing the parameters that I used for the simulation below:title = CHARMM OIL WATER NPAT RUN FOR IFT
; Run parameters
integrator = md ; leap-frog integrator
nsteps = 20000000 ; 2 * 50000 = 100 ps
dt = 0.002 ; 2 fs
; Output control
nstxout = 100000 ; save coordinates every 1.0 ps
nstvout = 100000 ; save velocities every 1.0 ps
nstenergy = 100000 ; save energies every 1.0 ps
nstlog = 100000 ; update log file every 1.0 ps
; Bond parameters
continuation = yes ; Restarting after NVT
constraint_algorithm = lincs ; holonomic constraints
constraints = h-bonds ; bonds involving H are constrained
lincs_iter = 1 ; accuracy of LINCS
lincs_order = 4 ; also related to accuracy
; Nonbonded settings
cutoff-scheme = Verlet ; Buffered neighbor searching
ns_type = grid ; search neighboring grid cells
nstlist = 10 ; 20 fs, largely irrelevant with Verlet scheme
rcoulomb = 1.2 ; short-range electrostatic cutoff (in nm)
rvdw = 1.2 ; short-range van der Waals cutoff (in nm)
DispCorr = EnerPres ; account for cut-off vdW scheme
; Electrostatics
coulombtype = PME ; Particle Mesh Ewald for long-range electrostatics
pme_order = 4 ; cubic interpolation
fourierspacing = 0.16 ; grid spacing for FFT
; Temperature coupling is on
tcoupl = V-rescale ; modified Berendsen thermostat
tc-grps = System ; two coupling groups - more accurate
tau_t = 0.1 ; time constant, in ps
ref_t = 298 ; reference temperature, one for each group, in K
; Pressure coupling is on
pcoupl = Berendsen ; Pressure coupling on in NPT
pcoupltype = semiisotropic ; uniform scaling of box vectors
tau_p = 1.0 ; time constant, in ps
ref_p = 1.0 1.0 ; reference pressure, in bar
compressibility = 0 4.5e-5 ; isothermal compressibility of water, bar^-1
refcoord_scaling = com
; Periodic boundary conditions
pbc = xyz ; 3-D PBC
; Velocity generation
gen_vel = no ; Velocity generation is off