[Pw_forum] equivalent supercells, vdW, stresses

[stefano de gironcoli]

Dear Niels,

are you sure the two cells are equivalent?
Are the results of the two cells still different if you remove the 
dipole correction?
In your message you say the in-plane stresses are different, and in the 
subsequent line you say they are equal... What exactly is different and 
what is equal?

best,
   stefano

On 02/09/2015 10:17, Niels Walet wrote:
> I have tried to run a geometry optimisation on a 2D water slab using 
> VDW-DF exchange functional, dipole correction, etc., etc.
>
> I find that by starting from different initial states, I can end up 
> with two equivalent supercells, one of which has a rectangular basis, 
> and the other has a parallelogram as base (angle about 60 degrees). 
> The total energies after minimisation agree to 7 decimal places. So 
> far so good. The in-plane stresses are very different, however: the 
> rectangular one is 2.7 GPa, and the parallelogram one is 2.2 GPa (both 
> in plane stresses are equal as one would expect).
>
> Is there any reason why that should be the case? Anyone aware of any 
> pitfalls with the calculation of stresses in the vdW part of the code? 
> I am using QE 5.1.2.
>
> Niels
>
> ---
> Prof. Niels R. Walet Phone:  +44(0)1613063693
> School of Physics and Astronomy         Fax: +44(0)1613064303
> The University of Manchester               Mobile: +44(0)7516622121
> Manchester, M13 9PL,  UK                  room 7.7, Schuster Building
> email: Niels.Walet at manchester.ac.uk
> web: http://www.theory.physics.manchester.ac.uk/~mccsnrw 
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