Dear Mohammed
I've found 3.9 eV for Mn (with PAW PP) a few years ago for a very
similar layered Mn oxide, and applied the same value to Mn(IV) in
monoclinic Ca2Mn3O8 (J. Am. Chem. Soc. 2015, 137, 10254, see SI). This
said, if you want to have a better Eg you must apply the U correction
also to O 2p (you may want to read Phys. Rev. X 2015, 5, 011006, where
MnO2 is discussed in the framework of a "generalized U" method).
However, keep in mind two things:
1) When dealing with (almost) full bands such as that arising from O
2p in Mn oxides, you should not use linear response to calculate U,
and you should use some semiempirical fitting instead (if it pleases
you...).
2) If your fitting parameter is the band gap, then be careful to check
that you have a good position of *both* band edges, maybe by comparing
results obtained with the same PPs and same cell parameters with those
obtained by using, eg. the HSE functional. It may happen (it happened
to me in many cases where inter-site contributions as U+V could not be
avoided, like GaAs or Si) that the U correction lowers the VB edge
*and* the CB edge, maybe leading to a correct gap in a wrong overall
potential landscape.
HTH
Giuseppe
Quoting Lorenzo Paulatto :
I have calculated the cell parameters and band-gap energy by GGA
and GGA+U for CaMnO3 materiels (orthorhombic structure ). I tried
two U values, one from the literature (U1=4.18 eV) and I have
calculated the other one by the first principal method with the
quantum espresso package (U2=5.7722 eV ). The band-gap energies
whose I have calculated by GGA, GGA+U1 and GGA+U2 are 0.71eV, 0.80
eV and 0.88 eV respectively. However, these values are very small
in comparison to the experimental result Eg=3.07 eV.
Hello, I would proceed in two directions:
1. examine the band structure with projwfc, in order to determine
which is the character of the states close to the gap (es. Mn d, or
Oxygen O...). The applying a +U to those states can open the gap.
2. add an arbitrary but not absurd U to all species and see if I can
manage to open the gap in this way or if it is absolutely impossible
cheers
U2 was calculated by hp.x code in QE (version 6.5) package with
2*1*2 and 2*2*2 kpoints. In addition, 2*1*2 and 2*2*2 gave the
same U2 value .
you can find my input files below
--
*Best regards,*
-
*Mohammed Lamhani,* Ph.D.
/LS3M Laboratory, Polydisciplinary Faculty of Khouribga, Sultan
Moulay Slimane University of Beni Mellal, Morocco./
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GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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E-mail:
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