[Wannier] Wannier functions of molecular crystal

[Gianluca Giovannetti]

Dear Jonathan,

thank you for your reply.

I think the symmetry of the WFs should be of p character.

I have however one problem.
I have 4 bands.
The number of Carbon ions in one molecule is 22 (and there are 2 molecules).

How can i set the flags:

begin projections

end projections

in such case in order to avoid the code messagge: "param_get_projection: too
many projections defined"?

ciao

G.


On Tue, Apr 13, 2010 at 11:48 PM, Jonathan Yates <
jonathan.yates at materials.ox.ac.uk> wrote:

>
> On 13 Apr 2010, at 14:08, Gianluca Giovannetti wrote:
>
> Dear All,
>
> i`m calculating Wannier functions for a molecular crystal (sites are
> molecule with C and H ions within each molecule).
>
> i want to plot Wannier functions of the 4 bands at Fermi level.
> the system is metallic.
> these four bands come from HOMO and LUMO molecular states of the two
> inequivalent molecules in the unit cell.
>
>
> Gianluca,
>
>  I'm assuming that these four bands which span the fermi level form an
> isolated group (so no disentanglement is needed)
>
>
> My feeling is that the setting in the win file:
> begin projections
> random
> end projections
> is not really good as starting point.
> Indeed the CONV is 1.0E-03.
> HOMO and LUMO are localized on the molecules.
> What is the best strategy in this case?
>
>
> I think using randomly placed Gaussian functions is likely to be
> particularly bad in this case. If the orbitals are already fairly localised,
> and the cell is pretty large, then the trial functions might have little
> overlap with the bloch states. That won't make the calculation wrong - just
> that the starting point could be poor.
>  So if you think that the states are 2p like on Oxygen then try such
> projections as a starting guess. Sometimes a two-step procedure might help:
> look at the WF resulting from the random projections - if they appear to
> have some symmetry character then try again using a projection of that form.
>
>  It might help in this procedure to use a coarser kpoint mesh (4x4x4 or
> even 2x2x2) - I find that the minimiser can cope better with a poor starting
> guess in this case. Then when I know what the MLWF should look like, I
> increase the mesh.
>
> However i get Wannier eigenvalues which are not exactly the same as the one
> i have calculated Quantum espresso (the discrepancy is not big but finite at
> some k-points).
>
>
>  You're obtaining the band-structure by interpolation. At the kpoints which
> lie in the original (ab-initio) grid the energies will be reproduced
> exactly. At any other points it is an interpolation. Importantly the quality
> of the interpolation increases exponentially as the size of kpoint mesh is
> increased.
>  However, it is probably a good plan to get well localised wannier
> functions first - as this may improve the interpolation.
>
>  Jonathan
>
>
>
>
> --
> Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH,
> UK
> tel: +44 (0)1865 612797                http://users.ox.ac.uk/~oums0549/<http://users.ox.ac.uk/%7Eoums0549/>
>
>
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