[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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