<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Aug 28, 2006, at 7:43 AM, lan haiping wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><BR><BR> <DIV><SPAN class="gmail_quote">On 8/28/06, <B class="gmail_sendername">hailin yu</B> <<A href="mailto:yuhailin_79@msn.com">yuhailin_79@msn.com</A>> wrote:</SPAN> <BLOCKQUOTE class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">Dear all:<BR> I have some quesitons want to know. Can you tell me ? Thanks a lot!<BR>1).Recently, I had calculated Ag3N which has the anti-ReO3 strucuture ( i.e,<BR>has the same structure with Cu3N,PRB 53,12648) by using CASTEP and found<BR>it's mechnical stabel ( I find the elastic siffness coefficients of Ag3N<BR>satisfy the generalized elastic stability criteria). But when I calculated <BR>the phonon dispersion of Ag3N using PWSCF, I found there was a soft mode<BR>along G-X direcition (i.e the phonon frequency is negative). For a negative<BR>frequency appear, it means that the structure is not stable? If so, the <BR>results of CASTEP and PWSCF is confict? This quesiton is still confuzed me<BR>and I am eager to receive your reply. Thanks!</BLOCKQUOTE> <DIV> </DIV> <DIV> </DIV> <DIV> you should give your input setting of calculation</DIV><BR></DIV></BLOCKQUOTE><DIV><BR class="khtml-block-placeholder"></DIV>This question deserves a more general answer that does not depend on any specific input of any specific calculation. The answer is kind of trivial, once one fully understands the question, but I am afraid that Halin Yu has some problems with the question itself. Suppose you are at the saddle between two valleys in a mountain landscape. Forces acting on you are zero. Is the equilibrium stable or unstable? Well, it depends on the direction: if you keep climbing up to the top of one mountain nearby, you will say that the equilibrium is stable. If you roll downhill, you would say instead that the equilibrium is unstable. Suppose now that your map only indicates the path climbing up. You would positively conclude that the euilibrium is stable. This is the conclusion that you drew from your CASTEP calculation that state that the equilibrium is stable with respect to distortions which change the shape of the unit cell (elastic stability criteria). These criteria do not say anything about the stability with respect to a phonon distortion which, by definition, does not change the shape of the unit cell.</DIV><DIV><BR><BLOCKQUOTE type="cite"><DIV> <BLOCKQUOTE class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">2). For f-electronic materials (such as Pt), for their lattice dynamics<BR>properties, it is reasonable to calculate using PWSCF ? </BLOCKQUOTE> <DIV> </DIV> <DIV>as far as i know, there is no problem for pwscf to calculate metal's lattice dynamics ,</DIV> <DIV>since it is Pseudopotential -plane wave method..</DIV></DIV></BLOCKQUOTE><DIV><BR class="khtml-block-placeholder"></DIV>as long as you do things properly ...</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Stefano B.</DIV><DIV><BR><BLOCKQUOTE type="cite"><DIV> <DIV> </DIV> <DIV> </DIV> <DIV>regards,</DIV> <DIV> </DIV> <DIV>hai-ping</DIV><BR> <BLOCKQUOTE class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">Thanks!<BR><BR> yuhailin<BR><BR>_________________________________________________________________<BR>Express yourself instantly with MSN Messenger! Download today it's FREE! <BR><A href="http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/">http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/</A><BR><BR>_______________________________________________<BR>Pw_forum mailing list <BR><A href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</A><BR><A href="http://www.democritos.it/mailman/listinfo/pw_forum">http://www.democritos.it/mailman/listinfo/pw_forum</A><BR></BLOCKQUOTE></DIV><BR></BLOCKQUOTE></DIV><BR><DIV> <SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; 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