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

[Ivan Scivetti]

Dear Matthias,

Thanks for having answered. I will investigate a bit further on the 
cell_opt bit before getting back to you.
Best wishes


On Monday, February 18, 2019 at 10:18:46 AM UTC, Ivan Scivetti wrote:
>
> 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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