[Pw_forum] k point convergence of dielectric properties

[dario rocca]

Dear Users
I have an issue related to the convergence of the static dielectric  matrix
using the PH code. I have performed calculations
on bulk silicon using different k point meshes and I have obtained the
following results:

k grid              diagonal component of      number of k points in
                         the dielectric tensor           the irreducible
Brillouin zone

4*4*4                   23.668350065                        8
 6 *6*6                16.297485614                       16
8 *8*8                  14.044830694                       29
10 *10*10             13.288531964                     47
12*12*12              13.029602882                     72
16 *16*16             12.908820645                    145
20 *20*20             12.894538380                    256

k grid+ 1 1 1 shift              diagonal component of      number of k
points in
                                            the dielectric tensor
the irreducible Brillouin zone

4*4*4                                      13.840844632
                          10
6*6*6
12.997009732                           28
8*8*8
12.903849607                           60
10*10*10                               12.893711596
110
12*12*12                               12.892685597
182
16*16*16                               12.892482798
408
20*20*20                               12.892537346
770

I was surprised of the improvement in the convergence due to the shift of
the grid. I don't think this is related to the number of k points
in the IBZ (at least not exclusively).
I have observed a similar behavior in diamond.
The ground state energy convergence also benefits from the shift, but the
improvement is not so striking.
Does someone has any hint on why the shift of the grid improves the
calculation of the dielectric properties of silicon?
Thanks a lot
Dario Rocca, dept. of chemistry, UC Davis
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