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

[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

> Am 24.09.2019 um 16:27 schrieb Ramanish Singh <sing... at umn.edu>:
> 
> 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 <javascript:>>:
>> 
>> 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 <javascript:>
> +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
tdku... at mail.upb.de
+49/(0)5251/60-5726

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