Incorrect pressure at energy minimum with rVV10

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aaron_kaplan1
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Incorrect pressure at energy minimum with rVV10

#1 Post by aaron_kaplan1 » Thu Jun 10, 2021 9:01 pm

This concerns calculations with meta-GGAs + rVV10. Sample input/output files using SCAN + rVV10 to determine the equilibrium geometry of graphite are attached.

When relaxing the cell directly (ISIF = 3), VASP successfully finds a zero pressure state. However, if an equation-of-state fitting is performed first, followed by an ISIF = 4 relaxation (to refine the equilibrium geometry), my collaborators and I are finding that:

1. There is a substantial external pressure at the true energy minimum (-9.15 kB in the attached files)
2. The direct relaxation with ISIF = 3 gets close to the true energy minimum, but overestimates the minimum (the difference is small, about 0.002 eV)

For calculations with SCAN alone, the agreement is much closer, with zero pressure at the true energy minimum. We've found the same trend when using other meta-GGAs with rVV10. We can't test this with GGAs + rVV10, as this isn't supported currently in VASP, though please correct me if I'm wrong.

ENCUT is set sufficiently high (1200 eV) to minimize Pulay stresses during the ISIF=3 relaxation, and there should be negligible Pulay stress from ISIF = 4, since the cell volume is held fixed there.

It's also worth noting that if the final geometry of the ISIF = 3 calculation is used as input to an ISIF = 4 relaxation, this still finds a zero pressure state near the energy minimum, but not the true energy minimum.

Could this be a bug in the stress tensor elements calculated with rVV10?
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martin.schlipf
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Re: Incorrect pressure at energy minimum with rVV10

#2 Post by martin.schlipf » Mon Jun 14, 2021 7:44 am

Thank you for reporting this to us, I could reproduce the behavior you described and will investigate its origin.

martin.schlipf
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Re: Incorrect pressure at energy minimum with rVV10

#3 Post by martin.schlipf » Tue Jun 22, 2021 11:07 am

The bug is now confirmed and we are working with our collaborators to fix it. We added a warning to the vdW-dF section that stress in rVV10 is broken.

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