Dear Laurence Marks Sir,<div><br></div><div> Thank you very much for your reply.....yes the question may not have a very good answer....if in any calculation we are not getting the desired convergence, it may happen that:</div>
<div>(1) Our chosen functional is not appropriate for the system</div><div><br></div><div>(2) The system (say while trying to predict a new material!) may not be feasible at all..</div><div><br></div><div>Probably all we can say that if we are able to achieve desired convergence (say 0.0001) we can say that we have used the appropriate functional for the system and the system is (may be) feasible (at least theoretically!! ).</div>
<div><br></div><div>Sir please correct me if I am wrong in my concept.</div><div><br></div><div>with best regards,</div><div><br><div class="gmail_quote">On Thu, Jul 7, 2011 at 6:12 PM, Laurence Marks <span dir="ltr"><<a href="mailto:L-marks@northwestern.edu">L-marks@northwestern.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">2011/7/7 Shamik Chakrabarti <<a href="mailto:shamikiitkgp@gmail.com">shamikiitkgp@gmail.com</a>>:<br>
<div class="im">> Dear Peter Blaha Sir,<br>
> Indeed by increasing number of K points we got the<br>
> convergence. Sir I have now some basic queries on this topic. You have said<br>
> that<br>
> "sometimes you cannot reach (easily) arbitrary<br>
> convergence"<br>
> why in some cases we can not reach convergence up to our desired limit?...is<br>
> it the limitation of DFT?....or it means that the feasibility of the<br>
> solution is only up to the achieved convergence?<br>
<br>
</div>This is in fact a deep, and very good question, at least in my opinion.<br>
<br>
Unfortunately that does not mean that there is a good answer to it!<br>
<br>
With the perfect functional convergence should (I believe, others may<br>
disagree) always be good. With a very imperfect functional it is quite<br>
possible that a DFT calculation will not converge, i.e. it is<br>
unfeasible. Empirically many (but not all) metals do not converge well<br>
with small numbers of k-points, but some others do. Why....I do not<br>
understand as I cannot write down a mathematical analysis to explain<br>
this and do not believe that there is a formal analysis in the<br>
literature, it is just empirical knowledge (folklore).<br>
<br>
<br>
--<br>
Laurence Marks<br>
Department of Materials Science and Engineering<br>
MSE Rm 2036 Cook Hall<br>
2220 N Campus Drive<br>
Northwestern University<br>
Evanston, IL 60208, USA<br>
Tel: (847) 491-3996 Fax: (847) 491-7820<br>
email: L-marks at northwestern dot edu<br>
Web: <a href="http://www.numis.northwestern.edu" target="_blank">www.numis.northwestern.edu</a><br>
Chair, Commission on Electron Crystallography of IUCR<br>
<a href="http://www.numis.northwestern.edu/" target="_blank">www.numis.northwestern.edu/</a><br>
Research is to see what everybody else has seen, and to think what<br>
nobody else has thought<br>
Albert Szent-Gyorgi<br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br>Shamik Chakrabarti<br>Research Scholar<br>Dept. of Physics & Meteorology<br>Material Processing & Solid State Ionics Lab<br>IIT Kharagpur<br>Kharagpur 721302<br>
INDIA<br>
</div>