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Hi,

There is a suggestion I'd like to make regarding output of the Born effective charges which may cause slight confusion to some users, as of VASP 6.4. Currently, The there are multiple ways Born effective charge are being output, regarding tags LEPSILON, LCALCEPS and IBRION=7,8. Most importantly, regarding LEPSILON and LCALCEPS, The charges are printed by each direction of finite field applied. However for IBRION=7,8, the charges are printed by each direction of ion displacement.

The reason they differ is due to the definition of Born effective charges, which can be seen in the following pdf page 110, equation (5.5) (Thesis of Dr. Ghosez):
http://www.phythema.ulg.ac.be/webroot/m ... Ghosez.pdf

This makes the Born effective charge output regarding field direction x for example: Z*xx, Z*xy and Z*xz, whereas the charge output regarding atomic displacement direction x: Z*xx, Z*yx and Z*zx

Current output does not mention this subtle difference, and consequently outputs the Born effective tensor for each ion in the following order:
Z*xx Z*yx Z*zx
Z*xy Z*yy Z*zy
Z*xz Z*yz Z*zz

Whereas I believe most users would expect the tensor to be conventionally arranged in this way unless explicitly mentioned otherwise:
Z*xx Z*xy Z*xz
Z*yx Z*yy Z*yz
Z*zx Z*zy Z*zz

Explicit mention of this arrangement would help reduce confusion for users in case they need to differentiate Z*ab and Z*ba.

Hi,

Thank you for your suggestion. Could you give some more details in terms of the location in the OUTCAR file? Do you mean the final Born effective charges (image 1) or for each step (image 2)?

Image 1:
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Image 2:
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When it is clear, we will add the distinction to the wiki to add clarity for future users.

I understand your issue. py4VASP reads them differently from how they are outputted in the OUTCAR file. This is ambiguous and we will add a note on the wiki to clarify this. When it is written, we'll add a link here to that page.

Best,

Chris

Please find the Born effective charges wiki page here: https://www.vasp.at/wiki/index.php/Born ... ve_charges