[CP2K-user] [CP2K:19774] Printing stress tensor from an existing trajectory

[Léon Luntadila Lufungula]

Dear Matthias,

I've done some checking myself, by comparing my calculated value with that 
reported in the MD section of the output and the reported value in the 
output does seem to coincide with just the average of the diagonal elements 
of the stress tensor as outputted by MOTION/PRINT/STRESS, without dividing 
by the cell volume... The elements of the stress tensor are already given 
in bar so dividing by the cell volume would give units of bar/Å^3 and not 
bar. So is it correct that the pressure is just equal to the average of the 
diagonal elements or does one need to take into account the cell volume as 
you suggested?

Kind regards,
Léon

P.S. Thanks to all of you from CP2K (Matthias, Jürg, Matt, ...) for being 
so responsive this last week, it has really helped me in getting my 
calculations and analysis of the results up and running in a short time!

On Thursday 18 January 2024 at 09:59:27 UTC+1 Krack Matthias wrote:

> Dear Léon
>
>  
>
> yes, the pressure is calculated from the average of the three diagonal 
> elements divided by the cell volume.
>
>  
>
> Best
>
>  
>
> Matthias
>
>  
>
> *From: *cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Léon 
> Luntadila Lufungula <Leon.luntad... at uantwerpen.be>
> *Date: *Wednesday, 17 January 2024 at 13:47
> *To: *cp2k <cp... at googlegroups.com>
> *Subject: *Re: [CP2K:19768] Printing stress tensor from an existing 
> trajectory
>
> Dear Matthias,
>
>  
>
> Perhaps this is a stupid question, but I want to calculate the pressure 
> from the stress tensor to see if my NPT trajectory is keeping the pressure 
> constant along its trajectory and I'm a bit unsure about how to do this. Is 
> this just taking the average of the diagonal elements of the stress tensor 
> or is it more complex than this?
>
>  
>
> Kind regards,
>
> Léon
>
> On Friday 12 January 2024 at 12:25:29 UTC+1 Léon Luntadila Lufungula wrote:
>
> Dear Matthias,
>
>  
>
> Thanks for the quick reply, it wasn't a very expensive calculation so it 
> doesn't matter if I'll have to re-do it. I'll keep this in mind for the 
> future so that I don't forget this again.
>
>  
>
> Kind regards,
>
> Léon
>
> On Friday 12 January 2024 at 11:22:39 UTC+1 Krack Matthias wrote:
>
> Dear Léon
>
>  
>
> I am afraid, it is not possible to recover the stresses a posteriori from 
> a NpT run. The stresses obtained with REFTRAJ would miss the kinetic energy 
> contribution to the total stress tensor.
>
>  
>
> HTH
>
>  
>
> Matthias
>
>  
>
> *From: *cp... at googlegroups.com <cp... at googlegroups.com> on behalf of Léon 
> Luntadila Lufungula <Leon.luntad... at uantwerpen.be>
> *Date: *Friday, 12 January 2024 at 10:16
> *To: *cp2k <cp... at googlegroups.com>
> *Subject: *[CP2K:19730] Printing stress tensor from an existing trajectory
>
> Dear all,
>
>  
>
> I calculated an NPT trajectory and wanted to see how the pressure 
> fluctuated during the MD run, however, I forgot to add 
> the MOTION/PRINT/STRESS keyword so I don't have my stresses. Is there a way 
> to create the file with the stresses after the fact without re-doing the 
> whole trajectory? Perhaps with the REFTRAJ keyword? I don't have any 
> experience with it, but I think it might pose a solution if I read the 
> documentation...
>
>  
>
> Kind regards,
>
> Léon
>
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