<div>Dear P.M. Grant,</div>
<div> </div>
<div>Thank you for your useful presentation. No, The system I consider is an insulator.</div>
<div> </div>
<div>Even tr2_ph was set to 1e-12, the negative frequency still can not be eliminated. Maybe it must be further reduced as you have said. But it will spend more computational time.<br> </div>
<div>Regards,</div>
<div>C.H.<br> </div>
<div><span class="gmail_quote">On 11/2/07, <b class="gmail_sendername">Paul M. Grant</b> <<a href="mailto:w2agz@pacbell.net">w2agz@pacbell.net</a>> wrote:</span>
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<p><span style="FONT-SIZE: 11pt; COLOR: #1f497d">Dear C. H.,</span></p>
<p><span style="FONT-SIZE: 11pt; COLOR: #1f497d"> </span></p>
<p><span style="FONT-SIZE: 11pt; COLOR: #1f497d">It would be helpful if we knew a little bit more about your system. Is it a metal? How many irreducible representations?</span></p>
<p><span style="FONT-SIZE: 11pt; COLOR: #1f497d"> </span></p>
<p><span style="FONT-SIZE: 11pt; 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 onclick="return top.js.OpenExtLink(window,event,this)" href="http://www.w2agz.com/snxaps07.htm" target="_blank">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.
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<p><span style="FONT-SIZE: 11pt; COLOR: #1f497d"> </span></p>
<p><span style="FONT-SIZE: 11pt; 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).
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<p><span style="FONT-SIZE: 11pt; COLOR: #1f497d"> </span></p>
<p><span style="FONT-SIZE: 11pt; 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.</span></p>
<p><span style="FONT-SIZE: 11pt; COLOR: #1f497d"> </span></p>
<p><span style="FONT-SIZE: 11pt; COLOR: #1f497d">Regards, -Paul</span></p>
<p><b><i><span style="FONT-SIZE: 13.5pt; COLOR: #1f497d"></span></i></b><span style="COLOR: #1f497d"></span> </p></div></div></blockquote></div><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>============================