[Pw_forum] Negative phonon frequencies in SiO_2

IYAD AL-QASIR iyad.ne at gmail.com
Mon Jan 2 16:33:02 CET 2012


Hello,
I have a suggestion that might help, since we experienced this problem
before.
Try to relax the structure, and especially the* atomic positions* before
you start phonon calculations.
Based on your optimization results also you will have better idea about
your PP choices.

Good Luck
IYAD

2012/1/2 Гриша Гончаровский <eiklm at mail.ru>

> Dear QE users,
>
> I attempt to calculate phonon dispersion in alpha-quartz and keep getting
> the wrong result. Optical branches look rather plausible, but acoustic
> branches include some negative frequencies, moreover, one branch seems to
> be negative in the whole Brillouin zone. I tried to use different
> pseudopotentials, to select different q-grids, ecutwfc and ecutrho values,
> but result remains the same.
> My scf input:
>
> &control
>    calculation='scf',
>    restart_mode='from_scratch',
>    prefix='sio2',
>    pseudo_dir = '/home/grysha/espresso/pseudo/',
>    outdir='/home/grysha/espresso/tmp/'
>  /
>  &system
>    ibrav=4,
>    celldm(1)=9.289897331,
>    celldm(3)=1.099552482,
>    nat=  9, ntyp= 2,
>    ecutwfc = 20.0
>    ecutrho = 150.0
>  /
>  &electrons
>    electron_maxstep=1000
>    mixing_mode = 'plain'
>    mixing_beta = 0.7
>    conv_thr =  1.0d-8
>  /
> ATOMIC_SPECIES
>  Si  28.086  Si.pz-vbc.UPF
>  O   15.999  O.pz-rrkjus.UPF
> ATOMIC_POSITIONS crystal
>  Si   0.46990000   0.00000000   0.66666667
>  Si   0.00000000   0.46990000   0.33333333
>  Si  -0.46990000  -0.46990000   0.00000000
>  O    0.41410000   0.26810000   0.78540000
>  O   -0.26810000   0.14600000   0.45206667
>  O   -0.14600000  -0.41410000   0.11873333
>  O    0.26810000   0.41410000  -0.78540000
>  O   -0.41410000  -0.14600000  -0.11873333
>  O    0.14600000  -0.26810000  -0.45206667
> K_POINTS automatic
> 4 4 4 0 0 0
>
> Phonon input:
>
> phonons of SiO_2
>  &inputph
>  tr2_ph=1.0d-12,
>  prefix='sio2',
>  ldisp=.true.,
>  nq1=4, nq2=4, nq3=4
>  amass(1)=28.086,
>  amass(2)=15.999,
>  outdir='/home/grysha/espresso/tmp',
>  fildyn='sio2.dynFull',
>  /
>
> q2r input:
>
> &input
>   fildyn='sio2.dynFull', zasr='simple', flfrc='sio2444.fc'
>  /
>
>
> Trying to calculate dispersion e.g. along the Gamma --- A line with such
> input
>
> &input
>    asr='simple',  amass(1)=28.086, amass(2)=15.999,
>    flfrc='sio2444.fc', flfrq='sio2.freq'
>  /
>  11
>  0.0 0.0 0.000000  0.0
>  0.0 0.0 0.0866025  0.0
>  0.0 0.0 0.173205  0.0
>  0.0 0.0 0.259808  0.0
>  0.0 0.0 0.346410  0.0
>  0.0 0.0 0.433013  0.0
>  0.0 0.0 0.519615  0.0
>  0.0 0.0 0.606218  0.0
>  0.0 0.0 0.692820  0.0
>  0.0 0.0 0.779423  0.0
>  0.0 0.0 0.866025  0.0
>
> I obtain in the sio2.freq file
>
> &plot nbnd=  27, nks=  11 /
>            0.000000  0.000000  0.000000
>   -3.9029   -1.6909   -0.9482   55.4410   58.8549  184.0277
>  232.6261  232.8523  328.4843  362.4152  372.6477  373.7721
>  424.6636  425.2693  446.6598  518.9579  681.4918  682.2090
>  775.2395  775.6317  778.7409 1081.8532 1082.0740 1096.3594
>  1172.6020 1172.7318 1231.8798
>            0.000000  0.000000  0.086602
>  -27.8791  -24.0601   34.3374   57.7231   79.0905  185.5367
>  222.9925  242.2063  317.0109  365.2996  371.2517  382.6987
>  418.6292  430.0279  445.5397  521.3980  655.6902  706.8253
>  771.8412  778.2792  779.1211 1079.7256 1084.3283 1096.2590
>  1161.7487 1183.7450 1233.8392
>            0.000000  0.000000  0.173205
>  -42.4501   -9.1280   61.2822   73.4236  100.4741  191.4075
>  214.2298  250.4044  293.9354  359.5278  385.7910  392.7004
>  412.1738  433.0018  442.8980  527.3652  629.0604  729.1692
>  768.1371  779.8632  780.1202 1077.7929 1086.6829 1095.9627
>  1152.2769 1193.5893 1237.5794
>            0.000000  0.000000  0.259808
>  -52.8832   51.6657   77.3462   97.0335  116.7849  202.2120
>  207.1785  255.6953  266.4743  356.9882  398.4107  401.5138
>  405.7037  434.3569  440.0324  534.6626  602.8471  747.0288
>  764.9138  780.6564  780.8533 1076.3172 1088.9413 1095.4574
>  1144.7947 1202.0362 1238.6139
>            0.000000  0.000000  0.346410
>  -58.6717   74.3589   85.0705  128.2999  130.9468  201.2035
>  208.4521  243.0812  255.8998  358.7639  398.8673  406.8712
>  407.6452  435.0435  437.8321  542.7959  579.1644  758.8697
>  762.7038  780.9131  781.1744 1075.3784 1091.0091 1094.6777
>  1139.5788 1210.4478 1234.1293
>            0.000000  0.000000  0.433013
>  -60.8276   83.9302   87.2635  134.9317  167.8644  185.4667
>  197.4218  245.0537  249.7366  362.0234  393.8809  410.1527
>  410.7431  435.4336  436.7230  553.4463  560.5617  761.4133
>  764.3324  781.0491  781.1413 1074.9821 1092.7449 1093.5520
>  1137.1544 1220.0745 1225.2006
>            0.000000  0.000000  0.519615
>  -60.1379   80.5518   86.7839  132.6697  149.3933  198.7177
>  200.3888  241.4125  253.5269  360.6371  395.7978  409.1896
>  409.5822  435.2824  437.1175  547.6607  569.0894  761.4928
>  762.8666  780.9771  781.1909 1075.1131 1091.9347 1094.1553
>  1137.9753 1215.1177 1230.0095
>            0.000000  0.000000  0.606218
>  -56.3113   65.0439   82.0152  112.8013  123.3145  204.0657
>  207.4619  252.7666  256.6986  357.3738  402.2872  403.2411
>  404.9860  434.7431  438.8123  538.5747  590.4774  753.7985
>  763.6404  780.8314  781.0603 1075.7777 1090.0095 1095.1065
>  1141.8689 1206.1496 1237.0771
>            0.000000  0.000000  0.692820
>  -48.3077   33.4208   70.6393   84.0546  109.2574  196.4080
>  210.5025  253.5698  280.4439  357.7446  392.5521  397.3795
>  408.9672  433.8191  441.4193  530.9458  615.7606  738.7961
>  766.4268  780.3237  780.5636 1076.9885 1087.8329 1095.7382
>  1148.2613 1197.9403 1238.6959
>            0.000000  0.000000  0.779423
>  -35.4182  -25.8479   49.1690   64.6900   90.4259  187.7989
>  218.3935  246.5521  306.2412  362.0995  378.5373  387.7484
>  415.3698  431.7777  444.3207  524.1062  642.3589  718.4768
>  769.9587  779.2468  779.5653 1078.7078 1085.5161 1096.1347
>  1156.7679 1188.8888 1235.7641
>            0.000000  0.000000  0.866025
>  -17.2456  -13.7809   17.5251   53.9027   67.3275  184.3786
>  227.8145  237.5840  325.1353  365.1032  368.9991  377.8392
>  421.8521  427.7529  446.3640  519.5984  668.8173  694.5999
>  773.6796  776.9999  778.8399 1080.8109 1083.1472 1096.3340
>  1167.0735 1178.3041 1232.4106
>
> Single-phonon calculation at the A point also gives some negative
> frequencies:
>
>     omega(  1 -  1) =        -96.6  [cm-1]   --> A
>     omega(  2 -  2) =        -69.9  [cm-1]   --> A
>     omega(  3 -  3) =        -52.9  [cm-1]   --> A
>     omega(  4 -  4) =        -25.6  [cm-1]   --> A
>     omega(  5 -  5) =        -13.8  [cm-1]   --> A
>     omega(  6 -  6) =        169.3  [cm-1]   --> A
>     omega(  7 -  7) =        218.6  [cm-1]   --> A
>     omega(  8 -  8) =        227.9  [cm-1]   --> A
>     omega(  9 -  9) =        318.1  [cm-1]   --> A
>     omega( 10 - 10) =        360.1  [cm-1]   --> A
>     omega( 11 - 11) =        362.7  [cm-1]   --> A
>     omega( 12 - 12) =        371.0  [cm-1]   --> A
>     omega( 13 - 13) =        412.5  [cm-1]   --> A
>     omega( 14 - 14) =        419.1  [cm-1]   --> A
>     omega( 15 - 15) =        441.1  [cm-1]   --> A
>     omega( 16 - 16) =        524.0  [cm-1]   --> A
>     omega( 17 - 17) =        668.1  [cm-1]   --> A
>     omega( 18 - 18) =        694.6  [cm-1]   --> A
>     omega( 19 - 19) =        773.0  [cm-1]   --> A
>     omega( 20 - 20) =        776.0  [cm-1]   --> A
>     omega( 21 - 21) =        779.8  [cm-1]   --> A
>     omega( 22 - 22) =       1078.2  [cm-1]   --> A
>     omega( 23 - 23) =       1080.8  [cm-1]   --> A
>     omega( 24 - 24) =       1094.3  [cm-1]   --> A
>     omega( 25 - 25) =       1165.6  [cm-1]   --> A
>     omega( 26 - 26) =       1175.8  [cm-1]   --> A
>     omega( 27 - 27) =       1244.2  [cm-1]   --> A
>
> but I wonder that positive frequencies in these two cases are not
> significantly different.
> Where should I dig to avoid instability? I would be grateful for any
> suggestion!
>
> Regards
>
> Mikhail Goncharovski
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>



-- 
_______________________________
IYAD I. AL-QASIR, PhD
Research Associate

Department of Nuclear Engineering
North Carolina State University
Campus Box 7909
2500 Stinson Dr.
Raleigh, NC 27695-7909
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