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

[Luca Babetto]

Dear all,

I apologise for this duplicate message, but my previous post has gone unanswered and I don't know if it was just lost amongst the various threads, so I'm reposting the message. I hope this does not come across as disrespectful, I greatly appreciate all the work the volunteers in the DFTB+ community put in to help users resolve these issues and I am well aware that support is not "owed" to us but simply granted by your kind souls! 😄

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"
  }
}

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.

I tried rerunning the calculation with the recommended settings for the barostat TimeScale value (0.1 ps), the same result occurs, just an order of magnitude slower as I would expect.

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 as I've seen done in the literature a few times? Should I include dispersion corrections explicitly via DFT-D3? Any pointer is greatly appreciated, thank you!

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

Kind regards

Luca
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