[CP2K-user] [CP2K:11289] Convergence problems with DFT+U in NPT ensemble

[Krack Matthias (PSI)]

Dear Ivan

I am wondering, if you performed a CELL_OPT run for your system in advance to relax the cell for a given external pressure? It is difficult to guess from the information you provide what the problem is. A cell change can also cause, of course, a change of the ground state. Especially during the initial part of a NPT_F run quite large cell fluctuation can occur.

Matthias

From: cp... at googlegroups.com <cp... at googlegroups.com> On Behalf Of Ivan Scivetti
Sent: Montag, 18. Februar 2019 11:19
To: cp2k <cp... at googlegroups.com>
Subject: [CP2K:11289] Convergence problems with DFT+U in NPT ensemble

Dear CP2K community,

I am computing a transition metal oxide for which I am applying DFT+U corrections in a supercell composed of 384 atoms.
To analyse structural properties at room temperature (295K) and standard pressure (1 atm) I need to carry out MD simulations. Due to the complexity of the system (of hexagonal symmetry), I start with the NVT ensemble and eventually reach equilibration. Electronic convergence runs smoothly and efficiently with the OT scheme. Since I do not know the range of the supercell volumes that corresponds to a pressure of 1atm, I started from the experimental values but, as expected, I get average pressures of the order of 500 to 1000 bars depending on the composition of the system. I could keep trying with different volumes and approach to the desire 1 atm pressure. This task, however, is very demanding and rather cumbersome.
I then decided to consider the NPT_F ensemble. However, the electronic DFT+U energy starts increasing significantly during the first steps of the simulation and, eventually, convergence problems start to manifest due to the Mulliken population analysis, with a negative contribution to the DFT+U energy. I could also try the LOWDIN method for charges but forces are not available for this scheme. Thus, it seems that my settings for the NPT_F ensemble are not appropriate for this DFT+U simulation. I describe the relevant MD settings of the input file below:

&Motion
  &MD
    Timestep 0.5
    Steps 2000
    Ensemble NPT_F
    Temperature 300.0
    &Thermostat
       REGION massive
       TYPE CSVR
       &CSVR
         TIMECON [fs] 10.
       &END CSVR
    &End Thermostat

   &Barostat
     Pressure 1.0
     TIMECON [fs] 10.
   &End barostat

&End Motion

 The message I obtain is the following
 *** WARNING in dft_plus_u.F:1457 :: DFT+U energy contibution is negative ***
 *** possibly due to unphysical Mulliken charges. Check your input, if    ***
 *** this warning persists or try a different method!                     ***

Again, this problems does not arise with NVT simlation, only with NPT. I am wondering if I need of an extra flag in the motion/thermostat session to "tame" the MD run and the electronic convergence. I have carried out an extensive research in the web but it seems there is no previous record of such a type of simulation with CP2K.

Any help would be appreciated.
Thank you very much in advance.

Ivan
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