[Wien] two mBJ questions

[Pavel Ondračka]

Dear Wien2k mailing list,

I have two questions regarding the mBJ potential.

I'm working on some TiO2 based ternary oxide solid solutions. The
supercells were relaxed using PBE and than I'm using the mBJ to
calculate band gaps and optical properties. The known problem for TiO2
with PBE (but also with LDA and maybe even some hybrids) is that the
anatase is predicted as the most stable structure instead of rutile.
However I have noticed, that mBJ actually gets this right. So I was
wondering if this would be a good enough reason to use the mBJ total
energies in discussion of for example structural stability and similar
stuff for the TiO2 based solid solutions instead of the PBE ones. 

I am aware that this has been brought up multiple times on the mailing
list, however there seems to be some disagreement, with some post
implying the mBJ total energies (and total energy dependent quantities)
should not be used at all: http://www.mail-archive.com/wien%40zeus.theo
chem.tuwien.ac.at/msg08020.html 
and some other posts suggest that maybe in some special cases it can be
used:
http://www.mail-archive.com/wien%40zeus.theochem.tuwien.ac.at/msg08522.
html
http://www.mail-archive.com/wien%40zeus.theochem.tuwien.ac.at/msg08523.
html

What would be your opinion in this case?

Another question is regarding the mBJ-GGA. It has been mentioned
several times, that it is possible to use GGA instead of LDA for energy
functional in mBJ. So far I've always used the default mBJLDA and
applied it to structures relaxed using the PBE (as recommended in the
original mBJ article).

However I have been asked, if it maybe would be better to use mBJ with
PBE as energy functional, when I have structures relaxed with PBE? My
current understanding is that since the mBJ parameters were calibrated
using LDA as energy functional, to use it with some else I would need
to recalibrate (refit) the parameters. 

Is this understanding correct?

Best regards
Pavel Ondračka