[Pw_forum] equivalent supercells, vdW, stresses

[Niels Walet]

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

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Prof. Niels R. Walet                             Phone:  +44(0)1613063693
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