Hi all,

I have a few questions about calculating the macroscopic polarization in siesta:

  1.  I see that the units are given in Debye, which is a dipole moment. So is 
siesta really outputting the macroscopic polarization * unit cell volume?
  2.  From the manual, it seems like the formula used to calculate the 
polarization is only defined up to a quantum. Does anybody know if the output 
polarization are given modulo the quanta (like in abinit), or if it is not 
possible to tell which polarization branch they are on? I am doing a set of 
calculations for a particular system which does not have a well-defined 
reference state, so I can't use the Born charges to measure the change in 
polarization across my calculations. The best I can do is measure the change of 
the macroscopic polarization across my calculations, so I am wondering if it is 
guaranteed that they will be on the same branch.
  3.  Does anybody have advice for choosing a good polarization grid? I know I 
should converge my results with respect to this, but not sure what the best way 
to do this would be. Should I just converge the diagonal terms and then the 
off-diagonal terms? Any advice would be appreciated.


Daniel Bennett
SIESTA is supported by the Spanish Research Agency (AEI) and by the European 
H2020 MaX Centre of Excellence (http://www.max-centre.eu/)

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