Dear Ioan, I didn't get an answer here, but yes it seems like the polarization is actually a dipole moment, so you need to divide by the volume. And I managed to get converged results after a long testing of all of the different inputs.
What you did sounds sensible. But with siesta it seems like there is no guarantee that you will get results on a particular polarization branch (somebody please correct me if I am wrong). This is probably why the polarization seems large. What you should do first is calculate the quantum of polarization manually using the atomic positions and formal charges (check the output of siesta or pseudopotential file if you are not sure), and take each polarization modulo the quantum. After that you can look at changes in polarization with respect to the cubic phase, and hopefully the numbers will seem more reasonable. But I don't know anything about BFO, so somebody else might be able to give some more specific advice. Daniel ________________________________ From: siesta-l-requ...@uam.es <siesta-l-requ...@uam.es> on behalf of ionut ghitiu <ionutghi...@yahoo.com> Sent: 21 April 2022 09:38 To: email@example.com <firstname.lastname@example.org> Subject: Re: [SIESTA-L] A few quesitons about macroscopic polarization Hi Daniel, I was wondering whether you received any response to your questions regarding the calculation of macroscopic polarisation in siesta, and if so, could you be so kind to share it. I'm currently trying to calculate the polarisation of some BiFeO3 systems but the values I obtain are about four times larger than expected. These were obtained by converting the output from Debye to C*m and then dividing each component by the volume of the unit cell to get the polarisation components. Following this I subtracted the values obtained for the symmetrical structure from those of the real system and calculated the resultant polarisation. Kind regards, Ioan On Monday, March 28, 2022, 11:04:25 PM GMT+3, Daniel Bennett <db...@cantab.ac.uk> wrote: 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. Thanks, 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/)
-- SIESTA is supported by the Spanish Research Agency (AEI) and by the European H2020 MaX Centre of Excellence (http://www.max-centre.eu/)