[Pw_forum] negative frequencies during phonon calculations
Paul M. Grant
w2agz at pacbell.net
Fri Nov 2 03:24:51 CET 2007
Dear C. H.,
It would be helpful if we knew a little bit more about your system. Is it a
metal? How many irreducible representations?
The Eliashberg formalism for e-p interactions in metals involves a "double
delta" integration over the Fermi energy dispersion at k-whatever, depends
on the broadening (screening) parameter chosen, and converges
excruciatingly.repeat painfully.slow. As an example, see the following page
from my web site (http://www.w2agz.com/snxaps07.htm) which contains the APS
March Meeting talk I gave earlier this year on my (feeble) attempt to
compute the gamma-point coupling in monoclinic (24 IRs !), 300 millikelvin
superconductor, polysulfur nitride, (SN)x. I used a tr2_ph = 1e-11, and
you'll see the lowest two values of omega are negative (open my talk). The
computation took a day on a 4-year old dual Xeon box. I figure a "serious"
calculation would require tr2_ph = 1e-14 at least, or a computer on the
scale of a Blue Gene.
Sidebar for the ph.x developers: It would be neat to be able to set a
"suspension" mid-execution flag to be able to resume a long run later (maybe
even on another computer).
Having said all this, Quantum-Espresso is the only free package I know that
even attempts to calculate alpha-squared F(omega). Viva Italia.
Regards, -Paul
Paul M. Grant, PhD
Principal, W2AGZ Technologies
Visiting Scholar, Applied Physics, Stanford University
EPRI Science Fellow (Retired)
IBM Research Staff Member Emeritus
<mailto:w2agz at pacbell.net> w2agz at pacbell.net
<http://www.w2agz.com/> http://www.w2agz.com
From: pw_forum-bounces at pwscf.org [mailto:pw_forum-bounces at pwscf.org] On
Behalf Of C.H. Hu
Sent: Thursday, October 25, 2007 1:31 AM
To: PWSCF Forum
Subject: [Pw_forum] negative frequencies during phonon calculations
Dear Pwscf users,
I performed gamma-point phonon calculation for my system only including some
light elements. Some negative frequencies appear as following. The
structure has been optimized first and the nscf calculation was also
performed. Does the negative frequencies indicate the structural
instability? or how to eliminate these imaginary frequencies during my
following calculations?
=========
q = ( 0.000000000 0.000000000 0.000000000 )
**************************************************************************
omega( 1) = -3.014004 [THz] = -100.537026 [cm-1]
omega( 2) = -1.833969 [THz] = -61.175042 [cm-1]
omega( 3) = -1.568539 [THz] = -52.321189 [cm-1]
omega( 4) = -1.362118 [THz] = -45.435661 [cm-1]
omega( 5) = -0.979698 [THz] = -32.679419 [cm-1]
omega( 6) = 3.631786 [THz] = 121.144148 [cm-1]
omega( 7) = 4.565661 [THz] = 152.295057 [cm-1]
omega( 8) = 4.870748 [THz] = 162.471753 [cm-1]
omega( 9) = 5.926574 [THz] = 197.690558 [cm-1]
omega(10) = 6.692205 [THz] = 223.229420 [cm-1]
omega(11) = 6.751437 [THz] = 225.205202 [cm-1]
omega(12) = 7.152446 [THz] = 238.581500 [cm-1]
omega(13) = 7.290775 [THz] = 243.195689 [cm-1]
omega(14) = 7.583548 [THz] = 252.961623 [cm-1]
omega(15) = 7.848509 [THz] = 261.799816 [cm-1]
omega(16) = 8.216032 [THz] = 274.059149 [cm-1]
omega(17) = 8.450907 [THz] = 281.893782 [cm-1]
omega(18) = 8.617843 [THz] = 287.462209 [cm-1]
omega(19) = 8.874576 [THz] = 296.025963 [cm-1]
omega(20) = 9.106260 [THz] = 303.754152 [cm-1]
omega(21) = 9.776634 [THz] = 326.115571 [cm-1]
omega(22) = 9.846084 [THz] = 328.432188 [cm-1]
omega(23) = 10.441744 [THz] = 348.301414 [cm-1]
omega(24) = 10.901272 [THz] = 363.629712 [cm-1]
omega(25) = 11.120174 [THz] = 370.931537 [cm-1]
omega(26) = 11.296950 [THz] = 376.828190 [cm-1]
omega(27) = 11.744897 [THz] = 391.770187 [cm-1]
omega(28) = 11.823415 [THz] = 394.389308 [cm-1]
omega(29) = 12.284723 [THz] = 409.776965 [cm-1]
omega(30) = 12.879001 [THz] = 429.600078 [cm-1]
omega(31) = 13.104238 [THz] = 437.113223 [cm-1]
omega(32) = 13.295358 [THz] = 443.488354 [cm-1]
omega(33) = 13.386218 [THz] = 446.519146 [cm-1]
omega(34) = 13.536055 [THz] = 451.517199 [cm-1]
omega(35) = 13.571174 [THz] = 452.688634 [cm-1]
omega(36) = 14.064683 [THz] = 469.150452 [cm-1]
omega(37) = 14.556450 [THz] = 485.554120 [cm-1]
omega(38) = 14.667300 [THz] = 489.251729 [cm-1]
omega(39) = 15.110352 [THz] = 504.030430 [cm-1]
omega(40) = 15.395285 [THz] = 513.534849 [cm-1]
omega(41) = 16.043631 [THz] = 535.161476 [cm-1]
omega(42) = 16.936124 [THz] = 564.932033 [cm-1]
omega(43) = 18.733204 [THz] = 624.876561 [cm-1]
omega(44) = 18.876424 [THz] = 629.653928 [cm-1]
omega(45) = 19.992373 [THz] = 666.878205 [cm-1]
omega(46) = 21.505666 [THz] = 717.356559 [cm-1]
omega(47) = 21.617840 [THz] = 721.098322 [cm-1]
omega(48) = 22.309307 [THz] = 744.163333 [cm-1]
omega(49) = 23.662551 [THz] = 789.303000 [cm-1]
omega(50) = 24.061062 [THz] = 802.595969 [cm-1]
omega(51) = 24.064277 [THz] = 802.703198 [cm-1]
omega(52) = 25.648159 [THz] = 855.536168 [cm-1]
omega(53) = 26.862084 [THz] = 896.028637 [cm-1]
omega(54) = 26.984089 [THz] = 900.098318 [cm-1]
omega(55) = 28.413367 [THz] = 947.774196 [cm-1]
omega(56) = 28.483349 [THz] = 950.108578 [cm-1]
omega(57) = 28.903442 [THz] = 964.121470 [cm-1]
omega(58) = 29.425892 [THz] = 981.548629 [cm-1]
omega(59) = 29.797340 [THz] = 993.938893 [cm-1]
omega(60) = 30.194172 [THz] = 1007.175875 [cm-1]
omega(61) = 30.533468 [THz] = 1018.493632 [cm-1]
omega(62) = 30.805908 [THz] = 1027.581303 [cm-1]
omega(63) = 31.041973 [THz] = 1035.455634 [cm-1]
omega(64) = 32.094778 [THz] = 1070.573675 [cm-1]
omega(65) = 33.759643 [THz] = 1126.107965 [cm-1]
omega(66) = 33.969486 [THz] = 1133.107616 [cm-1]
omega(67) = 36.434092 [THz] = 1215.318565 [cm-1]
omega(68) = 36.726631 [THz] = 1225.076683 [cm-1]
omega(69) = 39.522021 [THz] = 1318.321478 [cm-1]
omega(70) = 39.623203 [THz] = 1321.696562 [cm-1]
omega(71) = 40.034020 [THz] = 1335.400026 [cm-1]
omega(72) = 40.044563 [THz] = 1335.751729 [cm-1]
omega(73) = 50.292137 [THz] = 1677.576257 [cm-1]
omega(74) = 50.298606 [THz] = 1677.792065 [cm-1]
omega(75) = 51.050051 [THz] = 1702.857719 [cm-1]
omega(76) = 51.176742 [THz] = 1707.083716 [cm-1]
omega(77) = 53.131701 [THz] = 1772.294552 [cm-1]
omega(78) = 53.564853 [THz] = 1786.743033 [cm-1]
omega(79) = 53.946325 [THz] = 1799.467683 [cm-1]
omega(80) = 54.547644 [THz] = 1819.525644 [cm-1]
omega(81) = 55.855114 [THz] = 1863.138433 [cm-1]
omega(82) = 56.009839 [THz] = 1868.299538 [cm-1]
omega(83) = 56.878920 [THz] = 1897.289172 [cm-1]
omega(84) = 56.962499 [THz] = 1900.077075 [cm-1]
omega(85) = 57.147227 [THz] = 1906.238980 [cm-1]
omega(86) = 57.294672 [THz] = 1911.157244 [cm-1]
omega(87) = 58.113524 [THz] = 1938.471399 [cm-1]
omega(88) = 58.186280 [THz] = 1940.898280 [cm-1]
omega(89) = 60.420046 [THz] = 2015.409202 [cm-1]
omega(90) = 60.431210 [THz] = 2015.781574 [cm-1]
**************************************************************************
=========
Regards,
C.H.
--
============================
C.H. Hu
Postdoctoral fellow
Chimie et Physico-Chimie appliquees
Institut Francais du Petrole (IFP)
Rueil-Malmaison, France
E_mail: chaohao.mse at gmail.com
============================
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