[CP2K-user] [CP2K:12255] Negative Pressure in NVT ensemble

Ramanish Singh sing... at umn.edu
Tue Sep 24 14:27:32 UTC 2019

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?

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
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