[Pw_forum] Cu100 WRONG PHONON FREQUENCIES

[Alcantara Ortigoza, Marisol]

Dear all,

I have done phonon calculations on Cu(100) with both PBE and PZ
pseudopotentials to compare them with other calculations (using LDA
pseudopotential, 11 layers) and experimental results. These previous
calculations are in good agreement with experiments and were done with a
different code. 

So, I did 11 layers c(1x1) using PZ and,  9 and 13 layers using PBE

The relaxation of the systems was in good agreement with both experiment
and the other calculation in both cases. The problems came with the
phonon calculations. 

==============================================================

X point

9 layers PBE

Highest phonon frequency : OK
Surface phonons (S1,S2, S6) are from 1-4 meV above the expected values,
S4 is OK.

13 layers PBE

Highest phonon frequency : 1 meV below expected value
Surface phonons (S1,S2,S4) are 2-3 meV above ( 2 meV  below) the
expected values, S6 is OK.

11 layers PZ

Highest phonon frequency : 2 meV above expected value
Surface phonons are all 2 meV above the expected values

===================================================================

M point

9 layers PBE

Highest phonon frequency : 2 meV above expected value
Surface phonons (S1,S2,L1) are from 1 meV above (below) the expected
values

13 layers PBE

Highest phonon frequency : 2 meV above expected value
Surface phonons are OK

11 layers PZ

Highest phonon frequency : 0.5 meV above expected value
Surface phonon S1 is 2 meV above the expected value.
=====================================================================

The relaxation of these systems were performed until the forces were
below ~0.0008 Ryd/au

K-point mesh 15x15x1
Ecutwfc = 30 for PBE (40 for PZ)
Ecutrho = 360 for PBE (480 for PZ)
Width of intersurface vacuum space =  14 A

The phonon calculations were perforemed with

  tr2_ph=1.0d-12,
  alpha_mix(1) = 0.3


So, I would like to know if you have done similar calculations and, if
that is the case, the results you have found. Otherwise, I would like to
know if from your experience with other systems you would expect one may
get wrong phonon frequencies in this case. Or, maybe, you might detect
something wrong within my calculation which could be problem.

Thank you very much.

Best regards.
Marisol Alcantara Ortigoza.