[Pw_forum] Strange phonon branches appear in phonon dispersion
Viet Bac
vietbacchemist at gmail.com
Thu Dec 13 07:04:16 CET 2007
>
> Dear Dr. Stefano and Dr. Mohr,
Thank you for your comments,
Actually I had successfully done phonon dispersion and EPC for Al bulk
structure with good results so from now I want to apply for the Al wires
structures. Firstly I calculate for 2 atom cell of Al with zigzag wires, and
I got 6 phonon modes along q-points and appear some negative frequencies, so
I tried to improve my calculation but always appear negative acoustic
frequencies (optical branch is no problem) and then I did with 4, 6 atom
cell and of course I got 3-n modes but my results still no good like I
expect. :(
To check my set up parameters I calculate the same procedure for Al 1D
linear chain with 1 and 2 atom cell with hopefully the phonon curves will be
the same as I studied theoretically in Solid States books, but it was not
:(. So should my calculation be wrong with this system. Could you give me
advice about my input parameters? Please keep your eyes my INPUT files (in
case of 2 atoms in zigzag chain)
Al.scf.fit.in
&SYSTEM
ibrav = 6,
celldm(1) = 39.994,
celldm(3) = 0.199762906,
nat = 2,
ntyp = 1,
ecutwfc = 25.D0 ,
ecutrho = 300,
occupations = 'smearing' ,
degauss = 0.05 ,
smearing = 'methfessel-paxton' ,
la2F = .true.
/
&ELECTRONS
conv_thr = 1.D-10 ,
mixing_beta = 0.7D0 ,
/
ATOMIC_SPECIES
Al 26.98000 Al.vbc.UPF
ATOMIC_POSITIONS angstrom
Al 0.000000000 1.517315647 2.138739361
Al 0.000000000 0.116702849 0.023699809
K_POINTS automatic
4 4 40 0 0 0
Al.scf.in
&SYSTEM
ibrav = 6,
celldm(1) = 39.994,
celldm(3) = 0.199762906,
nat = 2,
ntyp = 1,
ecutwfc = 25.D0 ,
ecutrho = 300,
occupations = 'smearing' ,
degauss = 0.05 ,
smearing = 'methfessel-paxton'
/
&ELECTRONS
conv_thr = 1.D-10 ,
mixing_beta = 0.7D0 ,
/
ATOMIC_SPECIES
Al 26.98000 Al.vbc.UPF
ATOMIC_POSITIONS angstrom
Al 0.000000000 1.517315647 2.138739361
Al 0.000000000 0.116702849 0.023699809
K_POINTS automatic
2 2 20 0 0 0
Al.elph.in
Electron-phonon coefficients for Alwire_2
&inputph
tr2_ph=1.0d-14,
prefix='al2_elph',
fildvscf='al2d',
filelph='matrix'
amass(1)=26.98,
outdir='/root/tmp/',
fildyn='al2m.dyn',
elph=.true.,
trans=.true.,
ldisp=.true.
nq1=1, nq2=1, nq3=10,
And hereafter 6 modes I got
&plot nbnd= 6, nks= 27 /
0.000000 0.000000 0.000000
-0.0855 -0.0450 0.0000 20.7938 196.2309 372.4942
0.000000 0.000000 0.100000
-13.1432 5.6657 5.7114 20.3098 196.7060 372.0899
0.000000 0.000000 0.200000
-25.6753 10.2550 10.5805 18.8468 198.1533 371.0008
0.000000 0.000000 0.300000
-37.6999 14.0728 14.1858 16.2544 200.5950 369.4869
0.000000 0.000000 0.400000
-48.8295 11.6034 16.4265 16.4576 203.9728 367.7201
0.000000 0.000000 0.500000
-58.7192 -5.6944 16.7453 17.6129 208.1190 365.6663
0.000000 0.000000 0.600000
-67.0895 -15.3952 14.9673 18.4203 212.8051 363.1793
0.000000 0.000000 0.700000
-73.7443 -21.7038 11.9605 19.3942 217.8126 360.1938
0.000000 0.000000 0.800000
-78.6120 -26.9948 9.8326 20.4642 222.9635 356.8134
0.000000 0.000000 0.900000
-81.7857 -31.7595 10.1665 21.2176 228.1041 353.2106
0.000000 0.000000 1.000000
-83.4878 -36.1149 10.5785 21.4924 233.0809 349.4591
0.000000 0.000000 1.100000
-83.9637 -39.9380 6.4409 21.5912 237.7521 345.4939
0.000000 0.000000 1.200000
-83.3549 -43.0700 -11.8614 21.8643 242.0400 341.2345
0.000000 0.000000 1.300000
-81.7148 -45.5109 -19.5363 22.1233 245.9755 336.7214
0.000000 0.000000 1.400000
-79.2450 -47.4002 -24.2602 21.7049 249.6731 332.1110
0.000000 0.000000 1.500000
-76.4698 -48.7989 -26.8527 20.1426 253.2412 327.5276
0.000000 0.000000 1.600000
-74.0708 -49.5494 -28.8342 17.6459 256.7020 322.9460
0.000000 0.000000 1.700000
-72.4259 -49.4368 -31.6947 14.8469 259.9856 318.2672
0.000000 0.000000 1.800000
-71.2114 -48.4948 -35.6608 11.6256 263.0044 313.5063
0.000000 0.000000 1.900000
-69.6799 -47.0750 -39.7963 4.2536 265.7429 308.8670
0.000000 0.000000 2.000000
-67.4586 -45.5010 -43.2503 -12.7026 268.2849 304.5960
0.000000 0.000000 2.100000
-64.9647 -46.1360 -43.6300 -19.3793 270.7903 300.7406
0.000000 0.000000 2.200000
-63.0506 -49.2247 -40.9136 -23.4729 273.4711 297.0578
0.000000 0.000000 2.300000
-62.1609 -52.8376 -37.0315 -25.3558 276.5514 293.2015
0.000000 0.000000 2.400000
-61.8563 -56.2311 -32.5370 -26.2996 280.1674 289.0264
0.000000 0.000000 2.500000
-61.6387 -57.8617 -29.3445 -27.6304 283.8161 285.1565
0.000000 0.000000 2.503000
-61.6384 -57.8633 -29.3391 -27.6341 283.8284 285.1440
Bac,
------------------------------
Phung Viet Bac
Division of Mathematical and Physical Science,
Graduate School of Natural Science and Technology,
Kanazawa University,
Kakuma, Kanazawa 920-1192, JAPAN
>
> your phonon branches are really strange...
> with two atoms I would expect 6 modes per each q-point
>
> stefano
>
>
>
> 6 branches for 2 atom are what one expects (in general 3n branches and 3
> acoustical and 3n-3 optical ones).
>
> However negative values indicate some error in your calculation, dependent
> on the magnitude and system.
> You can impose some sum-rules for the acoustic ones, to force them to be
> 0 at the Gamma point.
>
> Acoustic forces are harder to calculate (smaller forces). In your case I
> would guess up to 15 cm^1 should be ok for the acoustic ones.
> Negative optical ones should not occur.
>
> Best
> Marcel Mohr
>
>
>
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