reference position bug during dipole moment calculation?

[Phil G.]

Dear Matt and Juergen,

thank you for your replies and hints.

@Matt: because I want to calculate the bulk polarization, I have to use the 
periodic system. So the Berry phase method was used  (in the DFT/PRINT 
section under MOMENTS I used PERIODIC .TRUE.). Do you think that the result 
would be different if I use non-periodic setup as you suggested?

@Juerg: yes, exactly I get the same results (with very small distinctions 
of the dipole moments after using different reference points). But why is 
the value of the polarization (as calculated by the dipole moment per slab 
volume) much smaller than the experimentally obtained value?
I would like to add a remark (as in my first mail above) that the sign of 
Y-value of the reference point is opposite than I gave it (e.g. if I write 
Y=a_y /2 (user defined value), then in the dipole moment output file it 
looks so: Y= - a_y / 2 , with an opposite sign). Is there a bug?

Kind regards,

Phil


Am Donnerstag, 12. Juli 2018 16:33:06 UTC+2 schrieb Phil G.:
>
> Dear all,
>
> in order to get a bulk polarization of a ferroelectric slab system (2x2x1 
> slab with 3D periodic boundary condition) I let CP2K start to calculate the 
> dipole moments of the slab system (in fact the unit cell has the hexagonal 
> shape, but it does not matter if I cut off a piece of the large system of 
> the duplicates of the unit cell in xyz coordinate system and this cut piece 
> satisfy the 3D pbc in the xyz coordinate system?). In order to check the 
> influence of the choice of a reference point on the dipole moment 
> calculation, I let start several calculations with different reference 
> points, e.g. COAC, ZERO, user_defined -> reference_point x y z.
>
> If using the reference_point ZERO, then I notice an non-explainable 
> difference between the quantity of the dipole moments for up-polarized slab 
> and down-polarized slab (the slabs are the same, but are only 180 degree 
> rotated to each other) and it seems to be the false choice for the dipole 
> moment calculation.
> If I use COAC as the reference point, then the both dipole moment values 
> are almost of the same quantity (absolute value) in z direction, but of 
> opposite sign (which is correct). If using COM as the reference point, then 
> I am wondering why not really the center of the slab was used as expected 
> (consider the slab system with lengths a_x, a_y, and c_z along the three 
> x,y,z coordinate axes, so I expect the COM to be nearly at {a_x / 2, a_y / 
> 2, c_z / 2}, but in the dipole moment file the reference point lies outside 
> of the slab system. Why? 
> If using the reference_point USER_DEFINED, REFERENCE_POINT X Y Z , in 
> which X, Y, and Z are of an arbitrary choice, for example at the center of 
> the top slab surface (X=a_x / 2 , Y=a_y / 2, Z=0) and the z=0 is exactly 
> the surface and below it there is the 2x2x1 slab system (i.e. with negative 
> c_z lying in the bottom slab surface), then after calculation I get the 
> info from dipole moment file that the y value has the opposite sign. Is 
> there a bug in the cp2k during the dipole moment calculation?
>
> After obtaining the calculated dipole moment values, I am able to 
> calculate the bulk polarization if the z-dipole moment (which is much 
> larger than in x- and y- directions) is divided by the slab volume (2x2x1 
> unit cells in the x,y,z coordinate system). Is that correct? If yes, why 
> the value is much smaller compared to the experimental value at room 
> temperature? (e.g. the experimental value of the polarization is 0.71 
> C/m^2  [APL 12 (2009), 209]  and the calculated value is 0.042 C/m^2 , i.e. 
> it is at least one order of magnitude smaller).
> Is there any mistake I have calculated or should I take account anything 
> for correct polarization calculation?
>
> The input file, output and dipole moment file after 200 geometry 
> optimization steps are attached here below.
>
> Kind regards,
>
> Phil
>
>
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