Hello,<br>I have a suggestion that might help, since we experienced this problem before.<br>Try to relax the structure, and especially the<b> atomic positions</b> before you start phonon calculations. <br>Based on your optimization results also you will have better idea about your PP choices. <br>
<br>Good Luck<br>IYAD<br><br><div class="gmail_quote">2012/1/2 çÒÉÛÁ çÏÎÞÁÒÏ×ÓËÉÊ <span dir="ltr"><<a href="mailto:eiklm@mail.ru">eiklm@mail.ru</a>></span><br><blockquote class="gmail_quote" style="margin:0pt 0pt 0pt 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
Dear QE users,<br>
<br>
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.<br>
My scf input:<br>
<br>
&control<br>
š šcalculation='scf',<br>
š šrestart_mode='from_scratch',<br>
š šprefix='sio2',<br>
š špseudo_dir = '/home/grysha/espresso/pseudo/',<br>
š šoutdir='/home/grysha/espresso/tmp/'<br>
š/<br>
š&system<br>
š šibrav=4,<br>
š šcelldm(1)=<a href="tel:9.289897331" value="+19289897331">9.289897331</a>,<br>
š šcelldm(3)=1.099552482,<br>
š šnat= š9, ntyp= 2,<br>
š šecutwfc = 20.0<br>
š šecutrho = 150.0<br>
š/<br>
š&electrons<br>
š šelectron_maxstep=1000<br>
š šmixing_mode = 'plain'<br>
š šmixing_beta = 0.7<br>
š šconv_thr = š1.0d-8<br>
š/<br>
ATOMIC_SPECIES<br>
šSi š28.086 šSi.pz-vbc.UPF<br>
šO š 15.999 šO.pz-rrkjus.UPF<br>
ATOMIC_POSITIONS crystal<br>
šSi š 0.46990000 š 0.00000000 š 0.66666667<br>
šSi š 0.00000000 š 0.46990000 š 0.33333333<br>
šSi š-0.46990000 š-0.46990000 š 0.00000000<br>
šO š š0.41410000 š 0.26810000 š 0.78540000<br>
šO š -0.26810000 š 0.14600000 š 0.45206667<br>
šO š -0.14600000 š-0.41410000 š 0.11873333<br>
šO š š0.26810000 š 0.41410000 š-0.78540000<br>
šO š -0.41410000 š-0.14600000 š-0.11873333<br>
šO š š0.14600000 š-0.26810000 š-0.45206667<br>
K_POINTS automatic<br>
4 4 4 0 0 0<br>
<br>
Phonon input:<br>
<br>
phonons of SiO_2<br>
š&inputph<br>
štr2_ph=1.0d-12,<br>
šprefix='sio2',<br>
šldisp=.true.,<br>
šnq1=4, nq2=4, nq3=4<br>
šamass(1)=28.086,<br>
šamass(2)=15.999,<br>
šoutdir='/home/grysha/espresso/tmp',<br>
šfildyn='sio2.dynFull',<br>
š/<br>
<br>
q2r input:<br>
<br>
&input<br>
š fildyn='sio2.dynFull', zasr='simple', flfrc='sio2444.fc'<br>
š/<br>
<br>
<br>
Trying to calculate dispersion e.g. along the Gamma --- A line with such input<br>
<br>
&input<br>
š šasr='simple', šamass(1)=28.086, amass(2)=15.999,<br>
š šflfrc='sio2444.fc', flfrq='sio2.freq'<br>
š/<br>
š11<br>
š0.0 0.0 0.000000 š0.0<br>
š0.0 0.0 0.0866025 š0.0<br>
š0.0 0.0 0.173205 š0.0<br>
š0.0 0.0 0.259808 š0.0<br>
š0.0 0.0 0.346410 š0.0<br>
š0.0 0.0 0.433013 š0.0<br>
š0.0 0.0 0.519615 š0.0<br>
š0.0 0.0 0.606218 š0.0<br>
š0.0 0.0 0.692820 š0.0<br>
š0.0 0.0 0.779423 š0.0<br>
š0.0 0.0 0.866025 š0.0<br>
<br>
I obtain in the sio2.freq file<br>
<br>
&plot nbnd= š27, nks= š11 /<br>
š š š š š š0.000000 š0.000000 š0.000000<br>
š -3.9029 š -1.6909 š -0.9482 š 55.4410 š 58.8549 š184.0277<br>
š232.6261 š232.8523 š328.4843 š362.4152 š372.6477 š373.7721<br>
š424.6636 š425.2693 š446.6598 š518.9579 š681.4918 š682.2090<br>
š775.2395 š775.6317 š778.7409 1081.8532 1082.0740 1096.3594<br>
š1172.6020 1172.7318 1231.8798<br>
š š š š š š0.000000 š0.000000 š0.086602<br>
š-27.8791 š-24.0601 š 34.3374 š 57.7231 š 79.0905 š185.5367<br>
š222.9925 š242.2063 š317.0109 š365.2996 š371.2517 š382.6987<br>
š418.6292 š430.0279 š445.5397 š521.3980 š655.6902 š706.8253<br>
š771.8412 š778.2792 š779.1211 1079.7256 1084.3283 1096.2590<br>
š1161.7487 1183.7450 1233.8392<br>
š š š š š š0.000000 š0.000000 š0.173205<br>
š-42.4501 š -9.1280 š 61.2822 š 73.4236 š100.4741 š191.4075<br>
š214.2298 š250.4044 š293.9354 š359.5278 š385.7910 š392.7004<br>
š412.1738 š433.0018 š442.8980 š527.3652 š629.0604 š729.1692<br>
š768.1371 š779.8632 š780.1202 1077.7929 1086.6829 1095.9627<br>
š1152.2769 1193.5893 1237.5794<br>
š š š š š š0.000000 š0.000000 š0.259808<br>
š-52.8832 š 51.6657 š 77.3462 š 97.0335 š116.7849 š202.2120<br>
š207.1785 š255.6953 š266.4743 š356.9882 š398.4107 š401.5138<br>
š405.7037 š434.3569 š440.0324 š534.6626 š602.8471 š747.0288<br>
š764.9138 š780.6564 š780.8533 1076.3172 1088.9413 1095.4574<br>
š1144.7947 1202.0362 1238.6139<br>
š š š š š š0.000000 š0.000000 š0.346410<br>
š-58.6717 š 74.3589 š 85.0705 š128.2999 š130.9468 š201.2035<br>
š208.4521 š243.0812 š255.8998 š358.7639 š398.8673 š406.8712<br>
š407.6452 š435.0435 š437.8321 š542.7959 š579.1644 š758.8697<br>
š762.7038 š780.9131 š781.1744 1075.3784 1091.0091 1094.6777<br>
š1139.5788 1210.4478 1234.1293<br>
š š š š š š0.000000 š0.000000 š0.433013<br>
š-60.8276 š 83.9302 š 87.2635 š134.9317 š167.8644 š185.4667<br>
š197.4218 š245.0537 š249.7366 š362.0234 š393.8809 š410.1527<br>
š410.7431 š435.4336 š436.7230 š553.4463 š560.5617 š761.4133<br>
š764.3324 š781.0491 š781.1413 1074.9821 1092.7449 1093.5520<br>
š1137.1544 1220.0745 1225.2006<br>
š š š š š š0.000000 š0.000000 š0.519615<br>
š-60.1379 š 80.5518 š 86.7839 š132.6697 š149.3933 š198.7177<br>
š200.3888 š241.4125 š253.5269 š360.6371 š395.7978 š409.1896<br>
š409.5822 š435.2824 š437.1175 š547.6607 š569.0894 š761.4928<br>
š762.8666 š780.9771 š781.1909 1075.1131 1091.9347 1094.1553<br>
š1137.9753 1215.1177 1230.0095<br>
š š š š š š0.000000 š0.000000 š0.606218<br>
š-56.3113 š 65.0439 š 82.0152 š112.8013 š123.3145 š204.0657<br>
š207.4619 š252.7666 š256.6986 š357.3738 š402.2872 š403.2411<br>
š404.9860 š434.7431 š438.8123 š538.5747 š590.4774 š753.7985<br>
š763.6404 š780.8314 š781.0603 1075.7777 1090.0095 1095.1065<br>
š1141.8689 1206.1496 1237.0771<br>
š š š š š š0.000000 š0.000000 š0.692820<br>
š-48.3077 š 33.4208 š 70.6393 š 84.0546 š109.2574 š196.4080<br>
š210.5025 š253.5698 š280.4439 š357.7446 š392.5521 š397.3795<br>
š408.9672 š433.8191 š441.4193 š530.9458 š615.7606 š738.7961<br>
š766.4268 š780.3237 š780.5636 1076.9885 1087.8329 1095.7382<br>
š1148.2613 1197.9403 1238.6959<br>
š š š š š š0.000000 š0.000000 š0.779423<br>
š-35.4182 š-25.8479 š 49.1690 š 64.6900 š 90.4259 š187.7989<br>
š218.3935 š246.5521 š306.2412 š362.0995 š378.5373 š387.7484<br>
š415.3698 š431.7777 š444.3207 š524.1062 š642.3589 š718.4768<br>
š769.9587 š779.2468 š779.5653 1078.7078 1085.5161 1096.1347<br>
š1156.7679 1188.8888 1235.7641<br>
š š š š š š0.000000 š0.000000 š0.866025<br>
š-17.2456 š-13.7809 š 17.5251 š 53.9027 š 67.3275 š184.3786<br>
š227.8145 š237.5840 š325.1353 š365.1032 š368.9991 š377.8392<br>
š421.8521 š427.7529 š446.3640 š519.5984 š668.8173 š694.5999<br>
š773.6796 š776.9999 š778.8399 1080.8109 1083.1472 1096.3340<br>
š1167.0735 1178.3041 1232.4106<br>
<br>
Single-phonon calculation at the A point also gives some negative frequencies:<br>
<br>
š š omega( š1 - š1) = š š š š-96.6 š[cm-1] š --> A<br>
š š omega( š2 - š2) = š š š š-69.9 š[cm-1] š --> A<br>
š š omega( š3 - š3) = š š š š-52.9 š[cm-1] š --> A<br>
š š omega( š4 - š4) = š š š š-25.6 š[cm-1] š --> A<br>
š š omega( š5 - š5) = š š š š-13.8 š[cm-1] š --> A<br>
š š omega( š6 - š6) = š š š š169.3 š[cm-1] š --> A<br>
š š omega( š7 - š7) = š š š š218.6 š[cm-1] š --> A<br>
š š omega( š8 - š8) = š š š š227.9 š[cm-1] š --> A<br>
š š omega( š9 - š9) = š š š š318.1 š[cm-1] š --> A<br>
š š omega( 10 - 10) = š š š š360.1 š[cm-1] š --> A<br>
š š omega( 11 - 11) = š š š š362.7 š[cm-1] š --> A<br>
š š omega( 12 - 12) = š š š š371.0 š[cm-1] š --> A<br>
š š omega( 13 - 13) = š š š š412.5 š[cm-1] š --> A<br>
š š omega( 14 - 14) = š š š š419.1 š[cm-1] š --> A<br>
š š omega( 15 - 15) = š š š š441.1 š[cm-1] š --> A<br>
š š omega( 16 - 16) = š š š š524.0 š[cm-1] š --> A<br>
š š omega( 17 - 17) = š š š š668.1 š[cm-1] š --> A<br>
š š omega( 18 - 18) = š š š š694.6 š[cm-1] š --> A<br>
š š omega( 19 - 19) = š š š š773.0 š[cm-1] š --> A<br>
š š omega( 20 - 20) = š š š š776.0 š[cm-1] š --> A<br>
š š omega( 21 - 21) = š š š š779.8 š[cm-1] š --> A<br>
š š omega( 22 - 22) = š š š 1078.2 š[cm-1] š --> A<br>
š š omega( 23 - 23) = š š š 1080.8 š[cm-1] š --> A<br>
š š omega( 24 - 24) = š š š 1094.3 š[cm-1] š --> A<br>
š š omega( 25 - 25) = š š š 1165.6 š[cm-1] š --> A<br>
š š omega( 26 - 26) = š š š 1175.8 š[cm-1] š --> A<br>
š š omega( 27 - 27) = š š š 1244.2 š[cm-1] š --> A<br>
<br>
but I wonder that positive frequencies in these two cases are not significantly different.<br>
Where should I dig to avoid instability? I would be grateful for any suggestion!<br>
<br>
Regards<br>
<br>
Mikhail Goncharovski<br>
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</blockquote></div><br><br clear="all"><br>-- <br>_______________________________<br>IYAD I. AL-QASIR, PhD<br>Research Associate <br><br>Department of Nuclear Engineering<br>North Carolina State University<br>Campus Box 7909<br>
2500 Stinson Dr.<br>Raleigh, NC 27695-7909<br>