Non-zero value of Born effective charge in metallic system

[raihan_ahammed]

Dear Admin and VASP users,

I have been trying to calculate the Born effective charge tensors (Z*) of a bulk metallic system using LEPSILON, with different k-point meshes and smearing settings. Surprisingly, I consistently obtain non-zero (Z*) tensors.

Based on my understanding of this previous discussion (viewtopic.php?t=20517&utm_source=chatgpt.com), I expected (Z*) to vanish for a metallic system, since the non-adiabatic method is not implemented in VASP. Is this interpretation correct, and how should I interpret the non-zero values?

Any clarification or suggestions would be greatly appreciated.

Kind regards,
Raihan

[martin.schlipf]

Your understanding is correct: There should be no Born effective charges in a metallic system. The way this shows up in practice is that the dielectric constant is huge and that your Born effective charges show an erratic convergence behavior. For an insulator you would expect to see a rapid convergence instead. Fundamentally it comes down to a linear response where you have the change of the Fermi occupancies in the numerator and the energy eigenvalues in the denominator. For an insulator the numerator is 1 (occupied - empty) and the denominator is the gap. For a metal both numerator and denominator approach zero and the results become numerically unstable.

As a side note, there are a few potential issues in your INCAR file. Please review that this is what you intended

• you set both ALGO and IALGO
• you set LREAL twice
• you calculate without spin polarization, are you sure Cr does not have a magnetic moment in this compound