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: [email protected]
web: http://www.theory.physics.manchester.ac.uk/~mccsnrw
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