[CP2K-user] [CP2K:12264] Negative Pressure in NVT ensemble

[Ramanish Singh]

Dear Prof. Thomas Kühne,

By relaxing the cell do you meaning doing a CELL_OPT operation? Or doing 
NPT simulations? Could you please elaborate on that?

And here's my D3 correction and SCF section: 

    &SCF

      SCF_GUESS ATOMIC ! can be used to RESTART an interrupted calculation

      MAX_SCF 30

      EPS_SCF 1.0E-6 ! accuracy of the SCF procedure typically 1.0E-6 - 
1.0E-7

      &OT

        ! an accurate preconditioner suitable also for larger systems

        PRECONDITIONER FULL_SINGLE_INVERSE

        ! the most robust choice (DIIS might sometimes be faster, but not 
as stable).

        MINIMIZER DIIS

      &END OT

      &OUTER_SCF ! repeat the inner SCF cycle 10 times

        MAX_SCF 10

        EPS_SCF 1.0E-6 ! must match the above

      &END


      &PRINT

        &RESTART OFF

        &END

      &END

    &END SCF


    ! specify the exchange and correlation treatment

    &XC

      

      &XC_FUNCTIONAL BLYP

      &END XC_FUNCTIONAL

      ! adding Grimme's D3 correction (by default without C9 terms)

      &VDW_POTENTIAL

         POTENTIAL_TYPE PAIR_POTENTIAL

         &PAIR_POTENTIAL

            PARAMETER_FILE_NAME dftd3.dat

            TYPE DFTD3

            REFERENCE_FUNCTIONAL BLYP

            R_CUTOFF [angstrom] 11.00

         &END

      &END VDW_POTENTIAL

    &END XC


Thanks,
Ramanish Singh


On Thursday, September 26, 2019 at 3:50:14 AM UTC-5, tkuehne wrote:
>
> Dear Ramanish Singh, 
>
> if I would know the answer I wouldn’t do any calculations anymore, but 
> rather use intuition instead ;) 
> However, I GUESS that your observation is not due to phase coexistence (I 
> have a lot to say here, 
> but would be way beyond the purpose of this mailing list), but more due to 
> an underestimation of the 
> density by DFT. Are you using any form of dispersion correction? Long 
> story short, beside the usage 
> of vdW correction, I recommend to relax the cell instead of over 
> pressurizing it. 
>
> Best, 
> Thomas Kühne
>
> Am 24.09.2019 um 16:27 schrieb Ramanish Singh <si... at umn.edu 
> <javascript:>>:
>
> Thanks Thomas Kühne. 
> I am simulating liquid Iodine close to the Vapor Liquid coexistence curve 
> using BLYP functional. I used experimental density to find the V for my NVT 
> simulations. But now I suspect there can be a two phase system in my box 
> and that's why the pressure is becoming negative. 
> So instead of the experimental density I am using slightly higher 
> density(~10 percent higher) to make sure it is in the liquid phase at that 
> Temperature. What are your thoughts on this?
>
> Thanks,
> Ramanish Singh
>
> On Monday, September 23, 2019 at 12:59:41 PM UTC-5, tkuehne wrote:
>>
>> Dear Ramanish Singh, 
>>
>> minimal mispredictions within lattice constant and vectors 
>> may entail large changes in the pressure. So observing a 
>> rather large negative pressure is not necessarily indicative 
>> of a problem. Beside that the usage of good basis sets and 
>> tight SCF convergence is essential, it is hard to say anything 
>> specific without detailed knowledge on what you plan to do. 
>>
>> Best, 
>> Thomas Kühne
>>
>> Am 23.09.2019 um 18:01 schrieb Ramanish Singh <si... at umn.edu>:
>>
>> Hi,
>>
>> I am simulating a system of Iodine atoms using BLYP functional in the NVT 
>> ensemble.
>> But when I am calculating the pressure, using   STRESS_TENSOR ANALYTICAL 
>> in the FORCE_EVAL section, my pressure (average of the xx,yy and zz 
>> elements) is coming out to be negative which is far from the experimental 
>> pressure at that density and Temperature. 
>> I can not figure out why this is happening. 
>>
>> Thanks,
>> Ramanish Singh
>>
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>> .
>>
>>
>> ==============================
>> Thomas D. Kühne
>> Dynamics of Condensed Matter
>> Chair of Theoretical Chemistry
>> University of Paderborn
>> Warburger Str. 100
>> D-33098 Paderborn
>> Germany
>> td... at mail.upb.de
>> +49/(0)5251/60-5726
>>
>>
> -- 
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> .
>
>
> ==============================
> Prof. Dr. Thomas D. Kühne
> Dynamics of Condensed Matter
> Chair of Theoretical Chemistry
> University of Paderborn
> Warburger Str. 100
> D-33098 Paderborn
> Germany
> td... at mail.upb.de <javascript:>
> +49/(0)5251/60-5726
>
>
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