[CP2K:5878] OT vs. diagonalization

[Ari Paavo Seitsonen]

Dear MP,

  My personal opinion: It depends on how large an error you can afford: If
you need to get the effects close to the Dirac cone "correctly" (well, you
might need a super cell of 60x60 or so), you cannot avoid the
diagonalisation because the electronic structure in the ground state has a
degeneracy exactly at the Fermi energy, and with OT you can only have one
of those states occupied, breaking the symmetry. The density of states,
however, is very small around the point K (or, the "Dirac point"), so a
small error in the states around the K point does not cause large errors in
many of the observables. If you have something that breaks the symmetry,
for example an adsorbate, a small gap is usually opened and in principle OT
might work. My general answer would be "it depends", one needs to test case
by case, but in the particular case of graphene I would expect that the
error made is not very large. Better to test 1) if you manage to converge
the electronic structure and 2) how large the difference with OT and
diagonalisation is in some cases.

    Greetings,

       apsi

2014-11-18 12:37 GMT+01:00 mpol <polynsk... at gmail.com>:

> Dear CP2k developers and users,
>
> Diagonalization instead of OT is suggested for KS wavefunction
> optimization in case of metals. But what about systems with small gap
> (semiconductors, transition metal clusters without ligands, etc.) or
> gapless systems (like e.g. graphene), can we use OT in these cases? Let's
> consider graphene, if one choses the supercell in a way that Dirac point is
> folded at gamma-point, the "gap" values may be small (~0.01 eV), even below
> the accuracy of currently available DFT methods. Can one use FULL_ALL
> preconditioner with ENERGY_GAP parameter equal to e.g. 0.0001 Ha in
> mentioned cases (small or zero gap)?
>
> With best regards,
> MP
>
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  Ari Paavo Seitsonen / Ari.P.S... at iki.fi / http://www.iki.fi/~apsi/
  Institut für Chemie der Universität Zürich
  Tel: +41 44 63 55 44 97  /  Mobile: +41 79 71 90 935
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