<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /></head><body style='font-size: 10pt; font-family: Verdana,Geneva,sans-serif'>
<p>Dear QE users,<br /><br />I have two fundamental questions regarding polarization calculated using the Berry Phase method.<br /><br />1. How should one convert the raw polarization printed in the nscf step from C/m2 to pC/m? <br /><br />For example, say for a monolayer system, the printed value of polarization is 1.0704332 (mod 1.0641566) C/m2, where the polarization quantum is Pq=1.0641566 C/m2. First, bring the raw polarization within the interval [-Pq/2,+Pq/2]. <br /><br />For this, I subtracted 1.0704332 - 1* 1.0641566 C/m2= 0.0062766 C/m2. Call it Pphysical.<br /><br />Now, to get to pC/m, what should I do? Should I multiply 0.0062766 C/m2 by the cell height 20 angstroms (assume the cell height is 20 angstroms) or assume a physical thickness of 3-5 angstroms? <br /><br />For a bilayer system, suppose the actual thickness of the slab is 3 angstrom (Zmax-Zmin=3 angstroms). Then physical thickness of the bilayer system will be roughly 6 angstroms (twice of 3 angstroms). so here should I multiply the polarization value (Pphysical: derived after bringing the raw polarization within [-Pq/2,+Pq/2]) by the cell height (assume cell height =20 angstroms) or should I multiply by the physical thickness of the slab (= 6 angstroms)??<br /><br />2. Secondly, Which is more correct approach to find polarization of a system : Planar averaged charged density difference analysis ( which gives only the electronic contribution) OR the Berry phase method which gives both the ionic and electronic components of the polarization? <br />This is important for heterobilayer systems. <br /><br />Kindly help me understand these points. <br /><br />Thanking you.<br /><br />Sincerely<br /><br />Jainandan Modi<br />Ph.D. Scholar<br />Electrical Engineering <br />IIT Kanpur<br />India</p>
</body></html>