[Wannier] parameter to control convergence

[Tadashi Ogitsu]

Dear Nicola,

Thanks for your quick reply. I'd like to develop chemical intuition  
of boron crystal from Wannier functions.

As you might know, boron chemistry is very unique; as it is described  
in many chemistry textbook, it does not follow conventional electron  
counting rule such as Lewis dot diagram (which works very well for  
carbon based systems). Typical example is B2H6 molecule, in which two  
hydrogen atoms are shared by two boron atoms forming two banana like  
bond (the other four hydrogen atoms seem to form the conventional  
bonds with boron atoms). I noticed that the Wannier function can be  
used to identify the banana bonds in boron system, e.g., the Wannier  
centre of the banana bond in B2H6 molecule indeed is located in the  
middle of triangle formed by two boron atoms and one hydrogen atom,  
contrary to a conventional covalent picture where the Wannier centre  
will be in the middle of two atoms (which could correspond to Lewis  
dot diagram). I'm trying to analyze the bonding nature of beta-boron  
crystal with this way and to compare with known empirical rule for  
boron chemistry such as mno rule.

I'll try your suggestions that to change the num_bands and num_wann,  
and let you know how it worked.

Meanwhile, I have a further question related to this issue.

Do the occupation given in the pwscf calculations used in Wannier90  
code? or in Wannier90, only eigenfunctions from the pwscf output are  
used and the energy window to construct the Wannier functions (or the  
definition of subspace for the unitary transformation?) is determined  
only by dis_*_[min|max] parameters? Thanks a lot!

#Using the occupation information does not sound making sense if I  
have an energy window, which specify the subspace, but as a beginner,  
I just wanted to make sure if I understand correctly.

Very best,
Tadashi




On Aug 23, 2006, at 6:54 AM, Nicola Marzari wrote:

>
>
> Dear Tadashi,
>
> welcome to the list ! It would be helpful to know what you'll need the
> MLWFs for - for chemical intuition, as a basis set, etc...
>
> My sense is that num_wann needs to capture the "natural"/"physical"
> number of orbitals that you want, and num_bands needs to be large  
> enough
> to make sure you capture all of the dispersive bands generated by the
> Bloch sums of the num_bands localized Wannier functions.
>
> To give you an example - suppose you were to study graphene (one
> sheet of graphite). Sensible num_bands would be 2.5 times the number
> of atoms, or 4 times the number of atoms.
>
> In the first case, you are expressing the desire to obtain MLWFs that
> represent bonding combination of sp^2 orbitals (i.e. the covalent  
> bonds
> of the graphitic backbone), and "half-empty" p_z orbitals. Each  
> atom will have 1 p_z orbital, and 3 covalent bonds - since each  
> bond is shared by 2 atoms, we have 1. + 3./2. = 2.5 MLWFs per atom  
> as a target.
>
> The other possibility would be 4 MLWFs per atom - then, you would  
> hope to disentangle 1 p_z and 3 sp^2 orbital per atom, all atom- 
> centered.
>
> If the electronic structure of beta-boron is similar to alpha  
> rombohedral, you need to ask yourself how the MLWFs of alpha
> look like - those are something that you want definitely to capture in
> your calculation (and you can use projection onto many different
> gaussians, to make sure you initialize the localization or  
> disentanglement correctly). What else you need to capture is the  
> tricky
> part, and some systems (e.g. graphite, or nanotubes) have empty states
> a few eV above Fermi that are very free-electron-like. You could start
> by playing around and increase num_bands - there should be a magical
> one in which everything falls into place.
>
> Worth a try would also be a num_bands equal to 210, and projection to
> s and p_x,y,z orbitals on each boron.
>
> Let us know,
>
> 			nicola
>
>
> 				nicola
>
>
> Tadashi Ogitsu wrote:
>> Hello,
>> I've just started to use the wannier90 code, and having a little  
>> problem in converging the Wannier functions, so, I would  
>> appreciate very much if you could give me a little bit of tips in  
>> controlling the convergence.
>> What I need is the Wannier centres and its spreads for beta- 
>> rhombohedral boron crystal. The system is a little bit big, 105  
>> boron atoms in the rhombohedral unit cell (this is not exactly the  
>> experimental structure but that is another story. So, let me call  
>> it *beta-boron* in this email). The electronic structure of the  
>> *beta-boron* is metalic so that I'm using the disentanglement  
>> scheme developed by SMV (thanks Ivo!). I've put 180 bands, and the  
>> Fermi level is at around 158th band (this is metal). So, I specify  
>> num_bands = 180 and num_wann = 158. I've also specified the  
>> dis_froz_max equal to the Fermi level given by the PWSCF 3.1.1.
>> The code seems to be working correctly, I mean, I do NOT see any  
>> warning (besides, "name = wann_main not found in io_stopwatch"  
>> when I restart the job from checkpoint), and the last message is  
>> "All done: wannier90 exiting".
>> I've made iterations up to a little over 300, and the individual  
>> spreads are certainly decreasing, mostly down to 1.4-1.5, looking  
>> like reasonable. DLTA parameters go down to 10^(-4). However,  
>> there are 4 wannier centres, whose spreads are unusually large,  
>> around 55, and they do not decrease.
>> So, my question is, which parameter shall I change to improve the  
>> convergence?
>> FYI: I've tested with alpha-rhombohedral boron, which consists of  
>> 12 boron atoms in the rhombohedral cell. This converged without  
>> any problem, and the results, i.e. the locations of Wannier  
>> centres and its spreads look perfectly reasonable. This system is  
>> an insulator so that it does not require the disentanglement  
>> procedure though.
>> I'm looking forward to hearing your suggestions. Thanks.
>> By the way, the system size is very large as you might notice, so,  
>> I think, it is a quite challenging problem but the code seems to  
>> be dealing very well (besides the convergence). I'm very impressed.
>> Best,
>> Tadashi Ogitsu
>> #I might be observing a memory leak but I'll post it when I get  
>> more accurate information if it comes from the computer or the code.
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>
>
> -- 
> ---------------------------------------------------------------------
> Prof Nicola Marzari   Department of Materials Science and Engineering
> 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
> tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
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