[DFTB-Plus-User] DFTB+ incorrect density in MD for water

[Luca Babetto]

Dear all,

I'm trying to run a MD simulation on a water box using the 3ob parameter set. I constructed a 200 molecules water box with packmol and I used the following input file to run the MD simulation:
Geometry = GenFormat {
  <<< "water_bulk.gen"
}

Driver = VelocityVerlet{
  TimeStep [fs] = 1.0
  Thermostat = NoseHoover {
    Temperature [Kelvin] = 298
    CouplingStrength [cm^-1] = 3200
  }
  Barostat = {
    Pressure [Pa] = 101325.0
    TimeScale [ps] = 0.01
  }
  Steps = 400000
  MovedAtoms = 1:-1
  MDRestartFrequency = 1000
}

Hamiltonian = DFTB {
  Scc = Yes
  SlaterKosterFiles = Type2FileNames {
    Prefix = "3ob/3ob-3-1/"
    Separator = "-"
    Suffix = ".skf"
  }
  kPointsAndWeights = { 0.0 0.0 0.0 1.0 }
  MaxAngularMomentum {
    O = "p"
    H = "s"
  }
}

Options {}

ParserOptions {
  ParserVersion = 11
}
I'm using periodic conditions with a 18.32 Angstrom box, for a starting cell volume of 6411.22 Angstrom^3 and a starting density of 973.75 g/L. I know I'm using a relatively low TimeScale value for the barostat, but I just wanted to quickly equilibrate the system and then run a production simulation.

My problem is that with these settings, the box keeps expanding and thus the density keeps decreasing. I stopped the simulation after 100ps because I saw that the volume kept expanding (linearly, with no sign of reaching an equilibrium value) until a final value of 6460.63 Angstrom^3, for a density of 926.06 g/L which is just too far away from the experimental value for me to use.

Am I using some incorrect settings? Is there a problem with the barostat, or maybe with the 3ob parametrization? Is DFTB simply not designed to reprocude the correct density for a water box so I should just set the "correct" volume and run NVT? Any pointer is greatly appreciated, thank you!

Let me know if you need more information on my calculation setup,

Kind regards

Luca

--


Luca Babetto, Ph.D.

Research Scientist


[cid:76cce7ff-d390-4fdd-8a1c-c2c647c3eb7d]

+39 0461 312360
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