[Wannier] xaxis

[Jonathan Yates]

On 12 Feb 2010, at 15:46, Gianluca Giovannetti wrote:
> however i want to have printed the H(R) in the file "_hr.dat" such  
> to have Wannier functions of Fe states with internal reference frame  
> directed along the Fe-Fe bonds (then pointing along o1-----2o bonds).
> Formally, if we want to maintain that e_g orbitals are x^2 - y^2 and  
> 3z^2 - r^2 and t_2g are xy, yz, zx then the above choice of  
> coordinate system gives
> the correct e_g - t_2g splitting coming from the Se crystal field  
> (e_g go lower in energy and t_2g higher).
>
> As my hamiltonian must be written in this set, my idea would be to  
> set x'=1,-1,0 in the file ".win".


The global x,y,z axes are set by your pw code. Wannier90 can use a  
projection of the bloch states onto a local function times spherical  
harmonics as an initial guesses for the MLWF. For example a p_z  
function. You might want the local coordinate system of this function  
to be different from the global x,y,z - in otherwords to have the p_z  
point in an arbitary direction. For this reason we allowed the  
possibility to specify the orientation for each projection.
  So what you did to align your starting guess functions is sound.

However, these functions are just starting guesses. The MLWF should be  
independent of these functions. No symmetry is imposed during the  
minimisation. Maybe the true MLWF do have the symmetry you want - in  
any case you can't automatically enforce that they have this symmetry.
  In practice the landscape can be complex, and you might find picking  
a 'good' starting guess helps to get to the true minimum. (if two  
different initial projections lead to different sets of WF, you can at  
least compare the spreads).

It might be worth looking at
Phys. Rev. B 65, 184422 (2002)
Maximally localized Wannier functions in antiferromagnetic MnO within  
the FLAPW formalism
  In this situation the minimizing the spread was not sufficient to  
uniquely define the MLWF - and a further step was used to obtain MLWF  
with the desired symmetry.


  Jonathan


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