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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Dear C. H.,<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>It would be helpful if we knew a little bit more about your
system. Is it a metal? How many irreducible representations?<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>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 (<a
href="http://www.w2agz.com/snxaps07.htm">http://www.w2agz.com/snxaps07.htm</a>)
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.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>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).<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Having said all this, Quantum-Espresso is the only free package
I know that even attempts to calculate alpha-squared F(omega). Viva
Italia.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Regards, -Paul<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><b><i><span style='font-size:13.5pt;font-family:"Comic Sans MS";
color:#1F497D'>Paul M. Grant, PhD</span></i></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:#1F497D'>Principal, W2AGZ Technologies</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:#1F497D'>Visiting Scholar, Applied Physics, Stanford University</span></b><span
style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:#1F497D'>EPRI Science Fellow (Retired)</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:#1F497D'>IBM Research Staff Member Emeritus</span></b><span
style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:#1F497D'><a href="mailto:w2agz@pacbell.net"><b><span style='color:#1F497D;
text-decoration:none'>w2agz@pacbell.net</span></b></a></span><span
style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Arial","sans-serif";
color:#1F497D'><a href="http://www.w2agz.com/"><b><span style='color:#1F497D;
text-decoration:none'>http://www.w2agz.com</span></b></a></span><span
style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'> <o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'> <o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span
style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>
pw_forum-bounces@pwscf.org [mailto:pw_forum-bounces@pwscf.org] <b>On Behalf Of </b>C.H.
Hu<br>
<b>Sent:</b> Thursday, October 25, 2007 1:31 AM<br>
<b>To:</b> PWSCF Forum<br>
<b>Subject:</b> [Pw_forum] negative frequencies during phonon calculations<o:p></o:p></span></p>
</div>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Dear Pwscf users,<br>
<br>
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?<br>
<br>
=========<br>
q = ( 0.000000000
0.000000000 0.000000000 )<br>
<br>
**************************************************************************<br>
omega( 1) = -3.014004
[THz] = -100.537026 [cm-1]<br>
omega( 2) = -1.833969
[THz] = -61.175042 [cm-1]<br>
omega( 3) = -1.568539
[THz] = -52.321189 [cm-1]<br>
omega( 4) = -1.362118
[THz] = -45.435661 [cm-1]<br>
omega( 5) = -0.979698
[THz] = -32.679419 [cm-1]<br>
omega( 6) =
3.631786 [THz] = 121.144148 [cm-1]<br>
omega( 7) =
4.565661 [THz] = 152.295057 [cm-1]<br>
omega( 8) =
4.870748 [THz] = 162.471753 [cm-1]<br>
omega( 9) =
5.926574 [THz] = 197.690558 [cm-1]<br>
omega(10) =
6.692205 [THz] = 223.229420 [cm-1]<br>
omega(11) =
6.751437 [THz] = 225.205202 [cm-1]<br>
omega(12) =
7.152446 [THz] = 238.581500 [cm-1]<br>
omega(13) =
7.290775 [THz] = 243.195689 [cm-1]<br>
omega(14) =
7.583548 [THz] = 252.961623 [cm-1]<br>
omega(15) =
7.848509 [THz] = 261.799816 [cm-1]<br>
omega(16) =
8.216032 [THz] = 274.059149 [cm-1]<br>
omega(17) =
8.450907 [THz] = 281.893782 [cm-1]<br>
omega(18) =
8.617843 [THz] = 287.462209 [cm-1]<br>
omega(19) =
8.874576 [THz] = 296.025963 [cm-1]<br>
omega(20) =
9.106260 [THz] = 303.754152 [cm-1]<br>
omega(21) =
9.776634 [THz] = 326.115571 [cm-1]<br>
omega(22) =
9.846084 [THz] = 328.432188 [cm-1]<br>
omega(23) = 10.441744
[THz] = 348.301414 [cm-1]<br>
omega(24) = 10.901272
[THz] = 363.629712 [cm-1]<br>
omega(25) = 11.120174
[THz] = 370.931537 [cm-1]<br>
omega(26) = 11.296950
[THz] = 376.828190 [cm-1]<br>
omega(27) = 11.744897
[THz] = 391.770187 [cm-1]<br>
omega(28) = 11.823415
[THz] = 394.389308 [cm-1]<br>
omega(29) = 12.284723
[THz] = 409.776965 [cm-1]<br>
omega(30) = 12.879001
[THz] = 429.600078 [cm-1]<br>
omega(31) = 13.104238
[THz] = 437.113223 [cm-1]<br>
omega(32) = 13.295358
[THz] = 443.488354 [cm-1]<br>
omega(33) = 13.386218
[THz] = 446.519146 [cm-1]<br>
omega(34) = 13.536055
[THz] = 451.517199 [cm-1]<br>
omega(35) = 13.571174
[THz] = 452.688634 [cm-1]<br>
omega(36) = 14.064683
[THz] = 469.150452 [cm-1]<br>
omega(37) = 14.556450
[THz] = 485.554120 [cm-1]<br>
omega(38) = 14.667300
[THz] = 489.251729 [cm-1]<br>
omega(39) = 15.110352 [THz]
= 504.030430 [cm-1]<br>
omega(40) = 15.395285
[THz] = 513.534849 [cm-1]<br>
omega(41) = 16.043631
[THz] = 535.161476 [cm-1]<br>
omega(42) = 16.936124
[THz] = 564.932033 [cm-1]<br>
omega(43) = 18.733204
[THz] = 624.876561 [cm-1]<br>
omega(44) = 18.876424
[THz] = 629.653928 [cm-1]<br>
omega(45) = 19.992373
[THz] = 666.878205 [cm-1]<br>
omega(46) = 21.505666
[THz] = 717.356559 [cm-1]<br>
omega(47) = 21.617840
[THz] = 721.098322 [cm-1]<br>
omega(48) = 22.309307
[THz] = 744.163333 [cm-1]<br>
omega(49) = 23.662551
[THz] = 789.303000 [cm-1]<br>
omega(50) = 24.061062
[THz] = 802.595969 [cm-1]<br>
omega(51) = 24.064277
[THz] = 802.703198 [cm-1]<br>
omega(52) = 25.648159
[THz] = 855.536168 [cm-1]<br>
omega(53) = 26.862084
[THz] = 896.028637 [cm-1]<br>
omega(54) = 26.984089
[THz] = 900.098318 [cm-1]<br>
omega(55) = 28.413367
[THz] = 947.774196 [cm-1]<br>
omega(56) = 28.483349
[THz] = 950.108578 [cm-1]<br>
omega(57) = 28.903442
[THz] = 964.121470 [cm-1]<br>
omega(58) = 29.425892
[THz] = 981.548629 [cm-1]<br>
omega(59) = 29.797340
[THz] = 993.938893 [cm-1]<br>
omega(60) = 30.194172
[THz] = 1007.175875 [cm-1]<br>
omega(61) = 30.533468
[THz] = 1018.493632 [cm-1]<br>
omega(62) = 30.805908
[THz] = 1027.581303 [cm-1]<br>
omega(63) = 31.041973
[THz] = 1035.455634 [cm-1]<br>
omega(64) = 32.094778
[THz] = 1070.573675 [cm-1]<br>
omega(65) = 33.759643
[THz] = 1126.107965 [cm-1]<br>
omega(66) = 33.969486
[THz] = 1133.107616 [cm-1]<br>
omega(67) = 36.434092
[THz] = 1215.318565 [cm-1]<br>
omega(68) = 36.726631
[THz] = 1225.076683 [cm-1]<br>
omega(69) = 39.522021
[THz] = 1318.321478 [cm-1]<br>
omega(70) = 39.623203
[THz] = 1321.696562 [cm-1]<br>
omega(71) = 40.034020
[THz] = 1335.400026 [cm-1]<br>
omega(72) = 40.044563 [THz]
= 1335.751729 [cm-1]<br>
omega(73) = 50.292137
[THz] = 1677.576257 [cm-1]<br>
omega(74) = 50.298606
[THz] = 1677.792065 [cm-1]<br>
omega(75) = 51.050051
[THz] = 1702.857719 [cm-1]<br>
omega(76) = 51.176742
[THz] = 1707.083716 [cm-1]<br>
omega(77) = 53.131701
[THz] = 1772.294552 [cm-1]<br>
omega(78) = 53.564853
[THz] = 1786.743033 [cm-1]<br>
omega(79) = 53.946325
[THz] = 1799.467683 [cm-1]<br>
omega(80) = 54.547644
[THz] = 1819.525644 [cm-1]<br>
omega(81) = 55.855114
[THz] = 1863.138433 [cm-1]<br>
omega(82) = 56.009839
[THz] = 1868.299538 [cm-1]<br>
omega(83) = 56.878920
[THz] = 1897.289172 [cm-1]<br>
omega(84) = 56.962499
[THz] = 1900.077075 [cm-1]<br>
omega(85) = 57.147227
[THz] = 1906.238980 [cm-1]<br>
omega(86) = 57.294672
[THz] = 1911.157244 [cm-1]<br>
omega(87) = 58.113524
[THz] = 1938.471399 [cm-1]<br>
omega(88) = 58.186280
[THz] = 1940.898280 [cm-1]<br>
omega(89) = 60.420046
[THz] = 2015.409202 [cm-1]<br>
omega(90) = 60.431210
[THz] = 2015.781574 [cm-1]<br>
**************************************************************************<br>
=========<br>
<br>
Regards,<br>
C.H.<br clear=all>
<br>
-- <br>
============================<br>
C.H. Hu<br>
Postdoctoral fellow<br>
Chimie et Physico-Chimie appliquees<br>
Institut Francais du Petrole (IFP)<br>
Rueil-Malmaison, France<br>
E_mail: <a href="mailto:chaohao.mse@gmail.com">chaohao.mse@gmail.com</a><br>
============================ <o:p></o:p></p>
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