[Wannier] Polarization of isolated H20 (dipole moment)

[Pedro Augusto F. P. Moreira]

  Dear all.

  I am trying to calculate the spontaneous electric polarization of a 
single water molecule as a test.
  I am using Quantum Espresso (pw.x and wannier90). I did by two 
methods:  (1) using  tefield=.true. as suggested by Cantele 
(http://www.democritos.it/pipermail/pw_forum/2009-March/011936.html) and 
(2) by Wannier function centres (WF).
  I run a set of simulations with different initial atomic positions. 
The first simulation, I used the positions mentioned by Cantele and 
found P = 1.82 D and P =1.84 D by method (1). By WF, P= 1.90 D and 1.92 
D. The first values are calculated with BLYP functional and the second 
ones with PBE functional.
  After that, I moved initially all atoms by a vector (1.0  0.0  0.0) 
angstrom, relaxed the molecule and did again the calculations above. I 
found for Cantele way: P = 1.78 D and 1.80 D. By WF, P = 6.33 D and 6.37 D.
  My questions are:

  (a) I expected to find values closer to 1.87 D in all calculations, 
but all they are diverging from this value by, at least, 2 %. Should I 
really expect it ?
  (b) Anyone knows what can be happening in WF calculations when I moved 
the atoms? I know that if the atoms were moved beyond the unit cell, a 
constant factor would be summed to polarization. But, I think that is 
not my case because the dislocation vector was 1.0 A , while my cubic 
cell has 10.6 A edges.

  With best regards,

  Pedro

-- 
Pedro Moreira

IFGW - Unicamp - Brazil