GGA tag (INCAR) and LEXCH tag (POTCAR)

[suresh0807]

Hello all,

I got a strange (may be obvious for some) behaviour while performing RPBE calculation using vasp 5.3.5.

I used a PBE pseudopotential and added the tag, 'GGA = RP', in the INCAR file.
From previous messages in the forum I came to know that I should ignore the warning (different XC in INCAR and POTCAR) and that
the GGA tag in INCAR replaces the default XC given in POTCAR (LEXCH).

However, I did a small test (may be a stupid one) and changed the LEXCH tag in the POTCAR file to 'RP' instead of default 'PE'.
As per my understanding, it should not alter the results since the default is set in INCAR (GGA = RP) tag.
But, I get different results.

1. Is this because the tag LEXCH in POTCAR is used to get the PAW reconstruction right during the calculation?
2. What are the differences in these two tags namely, GGA in INCAR and LEXCH in POTCAR and how do they influence the calculation?
3. Why does a PBE paw pseudopotential works well for a RPBE calculation? (I do understand that the only difference between PBE and RPBE is in its exchange part, but other than that I have no idea)

Thanks for your answers!!

[iciquser]

Hello,

I cannot answer the question exactly but I had a similar problem.
I was testing the PBEsol (GGA=PS) functional with and without changing LEXCH in POTCAR. When I changed the LEXCH-tag I got unreasonable values. So (also after discussing with my boss) I decided not to change the LEXCH tag in the POTCAR.

But if somebody knows more about is, I would also appreciate an answer to this.

Thanks

[suresh0807]

Hi iciquser,

Thanks for your inputs. I do hope that our questions will be answered soon.

Suresh