[Pw_forum] Negative phonon frequencies in SiO_2
Гриша Гончаровский
eiklm at mail.ru
Mon Jan 2 09:40:17 CET 2012
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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