[Wannier] disentanglement

[Jonathan Yates]

On 23 Mar 2010, at 17:44, Elie Moujaes wrote:

> Dear wannier90 users,
>  
> Please can anypne explain to me more about parameters like dis_win_max and dis_froz_min because I read the manual , i got confused and I did not get quite well what it was meant by these. Can these parameters for graphite be the same for graphene. Would one get good results if he did not take these parameters into account (i.e disentanglement involves all the MLWFs)

Firstly, you need to appreciate the need for disentanglement:
 For an isolated set of bands (eg valence bands of an insulator) we can directly apply the Marzari-Vanderbilt scheme for minimisating the spread of the wannier functions.
 However, for entangled bands (typically for metals or conduction states) we cannot do this directly and must first extract an optimal set of bands (according to the scheme of Souza, Marzari and Vanderbilt). Which means that at each kpoint we need to find a set of N bands which are some linear combination of the full set of bands. These N 'disentangled' bands are then used to form the N MLWF.


The inner energy window is used to select bloch states which will be included completely in the optimal set. This means that the MLWF will reproduce the true electronic properties at energies within the inner window,b ut outside there is no guarantee of this. So a common usage would be to place the inner window around the fermi energy - thus reproducing the fermi-surface properties. In the graphite example it has been chosen to reproduce all of the valence states, and a few eV above the Fermi energy.

 The outer window is less important. Try running the graphite example without it, and compare the bands.
   This is used to select a set of states out of which the optimal set can be chosen. One reason for doing this is to exclude any high energy bands which might have the same symmetry character as the lower states (and thus mix with them). Sometimes using an outer window can improve the convergence of the disentanglement procedure.

 Before you can set the windows you do need to know something about the bandstructure. One route is to look at the position of the fermi level from the scf calculation, and set the inner window a few eV above this. Obtain the MLWF and compare the bandstructure to the ab-initio one. Then you can decide (maybe by trial and error) to see if you need an outer window.

> Can these parameters for graphite be the same for graphene

 Probably. But it is better to understand why those values were chosen. There are examples in the tutorial, which could be modified to look at the effect of changing the window (eg try having no windows) - and also plenty of examples in the past literature - see http://www.wannier.org/papers.html - probably start with the CPC article listed first on that page which contains more details on the graphite example.


 Jonathan 


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