Subject: Mott insulators and DFT

From: "Artem R. Oganov" <[email-masked]>
Date: Sun, 25 Feb 2001 22:45:10 -0000

BlankDear CCLers,

I have the following question:

It is known that DFT within the LDA and GGA gives no band gap for such
transition metal oxides as FeO, CoO, NiO, which in fact are insulators (Mott
insulators). Are there any ideas about how serious this error is?

I suspect that there may be no real error: I understand that Kohn-Sham
eigenvalues and electronic density of states are, strictly speaking,
meaningless. Therefore, the absence of a band gap does not mean anything -
even more it does not mean that the material is a metal. There might be
other, more rigorous, ways to calculate the electrical conductivity from
DFT - but I am not aware of any such work. On the other hand, GGA and LDA
are known to reproduce many properties (elastic constants, equation of
state, magnetic ordering, magnetic moments) of these oxides really well - at
nearly the same level as they reproduce the properties, e.g., of MgO or
NaCl.

I would like to hear your ideas on:
1. The applicability of LDA/GGA to Mott insulators: can we use DFT at all,
what we can and what we cannot calculate.
2. If there is any error at all, how serious it is.
3.  How to calculate the electrical conductivity from DFT. Were such
calculations done for any of the Mott insulators?
4. What is the best way of treating Mott insulators, especially at high
pressures? LDA+U seems to be not too reliable at high pressures.
Hartree-Fock would definitely give a band gap - but it gives it even for the
free electron case! What else does exist?

I will summarise the replies on request.

Cheers,

Artem R. Oganov

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Artem R. Oganov
Department of Geological Sciences
University College London
Gower Street
London WC1E 6BT

tel: +44 (020)-7679-3344
fax: +44 (020)-7387-1612
email: [email-masked]
http://slamdunk.geol.ucl.ac.uk/~artem
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