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GGA and VOSKOWN use

1. If I use PAW_GGA POTCAR do I automatically use GGA or I have to put

GGA = PW/…

in INCAR additionally?

From Manual it looks like I should use GGA tag in INCAR only in special cases.


2. In manual I read:

"it is desirable to use this interpolation (Vosko Wilk and Nusair[46]) whenever the PW91 functional is applied."

I use PAW_GGA POTCARs. In there I found (for all elements I checked):
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LEXCH = 91
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Does "LEXCH = 91" mean that PW91 functional is applied when I use those POTCARs?

So in case of "LEXCH = 91" in POTCAR should I put VOSKOWN = 1 in INCAR?

Should I put also GGA = 91 in INCAR also?

1) the XC-algorithm is basically set by the type of POTCAR you use,
the GGA-tag only has to be given if some further specification is to be set
2) yes, LEXCH=91 means PW91
GGA=91 does not have to be set in INCAR in addition
3) VOSKOWN should generally be set if the PW91 functionals are used, it
HAS to be be set if magnetic systems are studied using PW91 potentials:
especially in the regime of onsetting magnetism, the magnetic moments
are enhanced due to the different parametrisation of the xc-contribution
of the free electron gas.