[QE-users] mod value in polarization calculation with Berry phase

[michel at if.usp.br]

Dear users and developers,

I calculated the electric polarization of two structures, Ca3Mn2O7 and  
Ca3Ti2O7, and I need some help to make sense of the results,  
particularly the polarization quantum.

At the output file for Ca3Ti2O7, where QE summarizes the results, it  
is printed a mod 2:

TOTAL PHASE: -0.53697 (mod 2)

In this case, the polarization quantum is twice what it should be  
(using the analytical expression). For Ca3Mn2O7, it prints mod 1 and  
the polarization is the same as the one calculated analytically.

What is this mod and why this diference? What is the correct  
polarization quantum of Ca3Ti2O7: the one printed by QE or this value  
divided by 2?

Thanks in advance for your help!

Michel L. Marcondes
Post-doctoral Research Scientist
University of Sao Paulo