[Wannier] parameter to control convergence

[Nicola Marzari]

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