[Pw_forum] equivalent supercells, vdW, stresses
stefano de gironcoli
degironc at sissa.it
Wed Sep 2 10:52:07 CEST 2015
are you sure the two cells are equivalent?
Are the results of the two cells still different if you remove the
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?
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.
> 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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