<div>Dear all,<BR>I want to calculate the phonon of diamond surface(001) which is terminated </div> <div>with H [C(001)-(2x1)-H]. I made a 10 layers C slab with 2 layers H on each </div> <div>side, then 8Ang vacuum. Actually I don't know that how thick should the slab </div> <div>and vacuum layer be.</div> <div>So I tested the phonon frequences of gamma point. At the beginning, I chose </div> <div><BR>ecutwfc = 40 <BR>ecutrho = 300<BR>nbnd = 80<BR><BR>diagonalization = 'david'<BR>mixing_mode = 'plain'<BR>mixing_beta = 0.7<BR>conv_thr = 1.0d-8<BR><BR>K_POINTS {automatic}<BR>4 2 1 0 0 0<BR><BR>tr2_ph = 1.0d-14,<BR><BR>the acoustic mode frequences(cm-1) were 40.288034, 42.596822, 65.242707 ;</div> <div>the highest optical mode frequences(C-H stretching) were 3859.619398, 3860.583787, 3874.784128 , 3877.700862.<BR><BR>Then I use the parameters:<BR><BR>ecutwfc = 50 <BR>ecutrho = 400<BR>nbnd = 80<BR><BR>diagonalization = 'david'<BR>mixing_mode = 'plain'<BR>mixing_beta =
0.7<BR>conv_thr = 1.0d-8<BR><BR>K_POINTS {automatic}<BR>4 2 1 0 0 0<BR><BR>tr2_ph = 1.0d-15,<BR><BR>the acoustic mode frequences(cm-1) were -56.563646, -38.181872, 66.896612 ;<BR>the highest optical mode frequences were 3856.407950 , 3857.530358, 3871.827365 , 3874.824054.<BR><BR>And then:<BR><BR>ecutwfc = 50 <BR>ecutrho = 800<BR>nbnd = 80<BR><BR>diagonalization = 'david'<BR>mixing_mode = 'plain'<BR>mixing_beta = 0.7<BR>conv_thr = 1.0d-8<BR><BR>K_POINTS {automatic}<BR>4 2 1 0 0 0<BR><BR>tr2_ph = 1.0d-14,<BR><BR>the acoustic mode frequences(cm-1) were 37.081314, 37.875120, 50.049387 ;</div> <div>the highest optical mode frequences were 3857.065094 , 3858.075173, 3871.875487, 3874.676259.<BR><BR>Evidently the ecutrho made the acoustic mode frequences tend to 0;<BR><BR>Question:<BR><BR>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?<BR><BR>2.How to determine the thickness of the slab and the thickness of the vacuum?<BR><BR>3.I relaxed all the atom in the slab, should I fixed the middle 4 layers (8 atoms)?<BR><BR>Thanks<BR>best regards!<BR></div> <div> </div> <div>Lijun Zhang</div><p>
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