My name is Anastasia Korovina. I am second year Master student at the Moscow Institute of Physics and Technology (MIPT) from Moscow.

Currently, I started to study ways for calculation of piezoelectric response of 2D materials using VASP and Quantum Espresso.

At this time, I carried out test calculations of piezoelectric coefficients for h-BN and MoS2. Unfortunately, obtained results are significantly different from reference. The value of the clamped-ion coefficients is the same as in reference data (3.7*10^-10 C/m), but the values for relaxed-ion are totally different than in reference value and equally the same comparing with clamped-ion approximation. Perhaps I misunderstand the origin of these approximations. I will be very grateful to you if you could please help me figure this out or could advise some person who can help?

For h-BN, we use rectangular unit cells of 4 atoms. Clamped-ion coefficients were achieved by setting of ISIF = 2, with no ionic relaxation (electronic only), so the ions cannot move to their position after deformation. Relaxed-ion coefficients were achieved by setting of ISIF = 2, but with the inclusion of electronic relaxation NSW = 900. However, this was not successful because I have got the same values for both configurations, although we clearly see in the literature that the values should be different. For the case of MoS2, we also used a rectangular unit cell (6 atoms). But in this case, I didn't obtain any correlation with the literature neither for clamped nor for relaxed-ion.

We used a several methods: we also replaced pseudopotentials for Mo atoms to more complete Mo_pv, set such flags as LCALCPOL, increased EDIFF, increased the number of k-points.

Now I am in a difficult situation, and I cannot move further in research. I would be very grateful if you would tell me where I am making a fundamental mistake.

Thanks in advance!

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