Stress-Strain-Curve fluctuating around zero

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GROMACS version: 2020.1-Ubuntu-2020.1-1
GROMACS modification: No

Hello,
I want to calculate the stress-strain-curve of a polymer, but evaluating “VIR-”, the potential Energy, or “PRESS-“ results in values that fluctuate around 0, with values ranging from e.g. +2000 [bar] to -2000 [bar], considering “PRESS-XX” (the sign sometimes switches from step to step). I was expecting the results for “PRESS-XX” to be negative in value but steadily increase (decrease), the graph resembling something more like a stress-strain-curve.

My general approach:

  1. Insert x-amount of polymers into a box (OPLS-AA forcefield)
  2. Energy minimization. See that the results become negative
  3. NVT (Position restrain all the polymers, constraint algorithm = lincs, constraints = h-bonds, tcouple = nose-hover, periodic boundary conditions pbc =xyz)
  4. NPT (Position restrain all the polymers, constraint algorithm = lincs, constraints = h-bonds, periodic boundary conditions pbc =xyz)
  5. Md-run:
    tcoupl = V-rescale
    Pcoupl = Parrinello-Rahman
    pcoupltype = semiisotropic
    tau_p = 1
    compressibility = 0 5e-7 5e-7 0 0 0
    ref_p = 0 1 1 0 0 0
    deform = 1e-7 0 0 0 0 0
    refcoord_scaling = com

I tried:

  • Different level of polymerization, ranging from 1 to 20 (I use a rather small polymer)
  • Different amounts of polymers inside the “box”
  • Not restraining the polymers in any step (nvt, npt)
  • Annealing
  • constraints = all-bonds
  • tcoupl = nose-hoover
  • Pcoupl = Berendsen
  • Different values for deform, compressibility and ref_p

Thank you for your suggestions in advanced.