[Pw_forum] How to determine the thickness in surface phonons calculation?

[Lijun Zhang]

Dear all,
I want to calculate the phonon of diamond surface(001) which is terminated 
  with H [C(001)-(2x1)-H]. I made a 10 layers C slab with 2 layers H on each 
  side, then 8Ang vacuum. Actually I don't know that how thick should the slab 
  and vacuum layer be.
  So I tested the phonon frequences of gamma point. At the beginning, I chose 
  
ecutwfc = 40 
ecutrho = 300
nbnd = 80

diagonalization = 'david'
mixing_mode = 'plain'
mixing_beta = 0.7
conv_thr = 1.0d-8

K_POINTS {automatic}
4 2 1 0 0 0

tr2_ph = 1.0d-14,

the acoustic mode frequences(cm-1) were 40.288034, 42.596822, 65.242707 ;
  the highest optical mode frequences(C-H stretching) were 3859.619398, 3860.583787, 3874.784128 , 3877.700862.

Then I use the parameters:

ecutwfc = 50 
ecutrho = 400
nbnd = 80

diagonalization = 'david'
mixing_mode = 'plain'
mixing_beta = 0.7
conv_thr = 1.0d-8

K_POINTS {automatic}
4 2 1 0 0 0

tr2_ph = 1.0d-15,

the acoustic mode frequences(cm-1) were -56.563646, -38.181872, 66.896612 ;
the highest optical mode frequences were 3856.407950 , 3857.530358, 3871.827365 , 3874.824054.

And then:

ecutwfc = 50 
ecutrho = 800
nbnd = 80

diagonalization = 'david'
mixing_mode = 'plain'
mixing_beta = 0.7
conv_thr = 1.0d-8

K_POINTS {automatic}
4 2 1 0 0 0

tr2_ph = 1.0d-14,

the acoustic mode frequences(cm-1) were 37.081314, 37.875120, 50.049387 ;
  the highest optical mode frequences were 3857.065094 , 3858.075173, 3871.875487, 3874.676259.

Evidently the ecutrho made the acoustic mode frequences tend to 0;

Question:

1.The highest mode should be about 3078 cm-1 (Th.Frauenheim Thin Solid Films 272 (1996) 314-330), while I don't know what makes the optical mode frequences so high, is it because of the thin vacuum layer?

2.How to determine the thickness of the slab and the thickness of the vacuum?

3.I relaxed all the atom in the slab, should I fixed the middle 4 layers (8 atoms)?

Thanks
best regards!

   
  Lijun Zhang

 		
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