Negative Pressure in NVT ensemble

[Ramanish Singh]

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

[Thomas Kühne]

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

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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

tdku...@mail.upb.de

+49/(0)5251/60-5726

[Ramanish Singh]

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 <sing...@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 Kühne]

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

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==============================

Prof. Dr. Thomas D. Kühne

[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

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