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I performed full structure optimisation (positions, cell volume and cell shape) using 15x15x3 k-mesh in VASP and safely assumed that the structure is reasonably relaxed as the external pressure found ~0.8 kB. When I took this relaxed structure (CONTCAR) to a different folder for SCF calculation (same k-points) with removed tags related to structure relaxation, I still found external pressure written in output but the value nearly unchanged. However, when I further used this structure for band structure calculation (with changed k-pts due to required KPATH) in a different folder, I found external pressure shoot up to ~52 kB. Do one need to get rid of this ext.pressure in non-SCF steps and if yes, please suggest a way to do so.

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The energy, pressure, forces etc are only accurate for a converged K-point grid.

When you do a band structure calculation, you are sampling a tiny subset of reciprocal space and e.g. pressure will necessarily be very different than the converged SCF calculation.

I suspect there is nothing wrong with your calculation and that is just an artifact. You could check by redoing the exact same NSCF calculation as you do for bands, but change the K-points to the full grid you used for SCF. I.e. calculate the density of states and check the pressure.

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