[QE-users] Polarization calculation using nscf

[Amar Singh]

Dear Lucian,
I managed to find your paper and its clears most of my doubts. However, I am still not able to understand the reason why the quantum of polarization (which you call Pq), is reducing to one fourth on a supercell of size eight times. Unlike P_FS or P_CS, Pq is independent of the "branch" picked up by the system and should remain invariant.
thanks

From: "Amar Singh"<amarsingh122014 at rediffmail.com>
Sent: Wed, 22 Jun 2022 18:16:26
To: "users"<users at lists.quantum-espresso.org>
Subject: Re: Polarization calculation using nscf

What I understood up to now is that the bracketed number (mode 0.5099 C/m2) is, what the reference calls the "quantum of polarization", and it depends upon the size and choice of the unit-cell selected. On the other hand, the number outside bracket is the "change in polarization", which is typically known as saturation polarization (Ps).Now, when I did some experiment with my unit-cell choice, the quantum of polarization 0.5 C/m2 for 2X2X2 supercell changed to 2 C/m2 (exactly four times) for 1X1X1 cell, whereas Ps changed from -0.16 C/m2 to 0.34 C/m2.I was expecting the quantum of polarization to either scale with the size of supercell or remain invariant, but here, on reducing the supercell volume to 1/8th, its value is quadrupled. On the other hand, change in polarization, which I was expecting to be independent of the choice of supercell size, has changed its magnitude as well as direction.I will really appreciate your help in understanding these numbers.
Thanks
 
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