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Dear Hailin Yu:<div><br class="webkit-block-placeholder"></div><div>didn?t you know that Quantum Espresso comes with a users' guide? Well, it is probably not as good as one can expect, and as we would like it to be, but it does contain the answer to ypur questions. See:</div><div><br class="webkit-block-placeholder"></div><div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font style="font: 12.0px Helvetica"><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;"><a href="http://www.pwscf.org/guide/3.1.1/other/users-guide-3.1.1.pdf">http://www.pwscf.org/guide/3.1.1/other/users-guide-3.1.1.pdf</a></span></font></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><br class="webkit-block-placeholder"></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">The above link was found by googling the following keywords:</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><br class="webkit-block-placeholder"></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font style="font: 12.0px Helvetica"><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;">"negative frequencies " site:pwscf.org</span></font></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><br class="webkit-block-placeholder"></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">This being said, the first calculation seems to be just affected by accuracy issues (that are briefly discussed in the users' guide referenced above). The second calculation, instead, seems to signal an instability of the optical modes 9such as a ferroelectric instability) that is not necessarily related to any elastic instability.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><br class="webkit-block-placeholder"></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Hope this helps</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Stefano</div> </div> <div><div><br class="webkit-block-placeholder"></div><div><br class="webkit-block-placeholder"></div><div><br class="webkit-block-placeholder"></div><div>On Oct 22, 2007, at 4:23 AM, hailin yu wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"> <br><br> <blockquote> <hr> From: <a href="mailto:yuhailin_79@msn.com">yuhailin_79@msn.com</a><br>To: <a href="mailto:pw_users@pwscf.org">pw_users@pwscf.org</a><br>Subject: Help! phonon calculation at G point.<br>Date: Wed, 17 Oct 2007 11:08:51 +0800<br><br> Dear all:<br> <br> I have some trouble with phonon calculation at G point. Recently, I have calculated two kinds of PtC, rosksalt structure and zinc-blende structure. Firstly, I have fully relaxed the lattice constants and atomic positions using quantum-espressso3.2.3. The results we attained is well agreement with these results using by VASP and CASTEP. Following, I calculate the phonon frequency at G point of the two structures. <br>For zinc-blende structure, we attained the phonon frequency as follows:<br> q = ( 0.000000000 0.000000000 0.000000000 )<br> **************************************************************************<br> omega( 1) = 1.086480 [THz] = 36.241319 [cm-1]<br> ( 0.340996 0.000000 0.272050 0.000000 -0.549845 0.000000 )<br> ;( 0.346068 0.000000 0.276096 0.000000 -0.558022 0.000000 )<br> omega( 2) = 1.086480 [THz] = 36.241319 [cm-1]<br> ( -0.191604 0.000000 -0.550378 0.000000 -0.391141 0.000000 )<br> ( -0.194454 0.000000 -0.558563 0.000000 -0.396958 0.000000 )<br> omega( 3) = 1.086480 [THz] = 36.241319 [cm-1]<br> ( 0.582777 0.000000 -0.340135 0.000000 0.193129 0.000000 )<br> ( 0.591444 0.000000 -0.345194 0.000000 0.196001 0.000000 )<br> omega( 4) = 16.225674 [THz] = 541.233828 [cm-1]<br> ( -0.026894 0.000000 0.048005 0.000000 0.029340 0.000000 )<br> ( 0. 430439 0.000000 -0.768321 0.000000 -0.469592 0.000000 )<br> omega( 5) = 16.225674 [THz] = 541.233828 [cm-1]<br> ( -0.036407 0.000000 0.009944 0.000000 -0.049641 0.000000 )<br> ( 0.582692 0.000000 -0.159157 0.000000 0.794513 0.000000 )<br> omega( 6) = 16.225674 [THz] = 541.233828 [cm-1]<br> ( 0.042894 0.000000 0.038539 0.000000 -0.023738 0.000000 )<br> ( -0.686516 0.000000 -0.616817 0.000000 0.379926 0.000000 )<br> **************************************************************************<br>I don't know why omega(1), omega(2) and omega(3) are not equal to zero? while the optic modes is well agreement with the results attained by VASP. I want to know how to improve the accuracy. Can you tell me? Thanks a lot!<br> <br>For roskslt structure, we attained the phonon frequency as follows:<br><br> q = ( 0.000000000 0.000000000 0.000000000 )<br> **************************************************************************<br> omega( 1) = -16.323420 [THz] = -544.494287 [cm-1]<br> ( 0.057067 0.000000 0.023583 0.000000 0.002155 0.000000 )<br> ( -0.921868 0.000000 -0.380961 0.000000 -0.034805 0.000000 )<br> omega( 2) = -16.323420 [THz] = -544.494287 [cm-1]<br> ( -0.015312 0.000000 0.032457 0.000000 0.050295 0.000000 )<br> ( 0.247345 0.000000 -0.524309 0.000000 -0.812467 0.000000 )<br> omega( 3) =&nbs p; -16.323420 [THz] = -544.494287 [cm-1]<br> ( 0.018065 0.000000 -0.046988 0.000000 0.035822 0.000000 )<br> ( -0.291827 0.000000 0.759046 0.000000 -0.578678 0.000000 )<br> omega( 4) = 0.284260 [THz] = 9.481955 [cm-1]<br> ( 0.369808 0.000000 -0.578558 0.000000 0.160509 0.000000 )<br> ( 0.371847 0.000000 -0.581748 0.000000 0.161394 0.000000 )<br> omega( 5) = 0.284260 [THz] = 9.481955 [cm-1]<br> ( 0.598650 0.000000 0.340881 0.000000 -0.150560 0.000000 )<br> ( 0.601951 0.000000 0.342761 0.000000 -0.151390 0.000000 )<br> omega( 6) = 0.284260 [THz] = 9.481955 [cm-1]<br> ( -0.045937 0.000000 -0.215223 0.000000 -0.669940 0.000000 )<br> ( -0.046191 0.000000 -0.216410 0.000000 -0.673634 0.000000 )<br> **************************************************************************<br>Why the phonon frequency is negative? As a test, I have caculated the elastic constants of this structure and find it satisfy formula:<br>C44>0, C11>|C12|,C11+2C12>0. So we don't know what was wrong. Can you tell me? Thanks!<br>The input file as shown in the follow:<br>RS.scf.in:<br> <br> &control<br> calculation='scf'<br> restart_mode='from_scratch'<br> tstress = .true.<br> tprnfor = .true.<br> prefix='RS'<br> etot_conv_thr=1.0D-5<br> forc_conv_thr=1.0D-4<br> & nbsp; pseudo_dir = '/home/yhl/espresso-3.0/pseudo/'<br> outdir='/home/yhl/tmp/'<br> /<br> &system<br> ibrav= 2, celldm(1) =8.98, nat= 2, ntyp= 2<br> ecutwfc =100.0<br> ecutrho =500.0<br> degauss =0.02<br> smearing='mp'<br> occupations ='smearing'<br> /<br> &electrons<br> conv_thr = 1.0d-12<br> mixing_beta = 0.7<br> /<br>ATOMIC_SPECIES<br> Pt 195.08 Pt.pbe-nd-rrkjus.UPF<br> C 12.01 C.pbe-rrkjus.UPF<br>ATOMIC_POSITIONS<br> Pt 0.00 0.00 0.00<br> C 0.5 0.5 0.5<br>K_POINTS AUTOMATIC<br> 12 12 12 1 1 1<br><br>RS.ph.in<br> <br> phonon of PtC<br>&inputph<br> tr2_ph=1.0d-14,<br> prefix='RS',<br> ldisp=.true.,<br> nq1=4,nq2=4,nq3=4<br> amass(1)=195.08,<br> amass(2)=12.01, <br> outdir='/home/yhl/tmp/',<br> fildyn='RS.dyn',<br> /<br><br><br> <hr> Discover the new Windows Vista <a href="http://search.msn.com/results.aspx?q=windows+vista&mkt=en-US&form=QBRE" target="_blank">Learn more!</a> </blockquote><br><hr>Discover the new Windows Vista <a href="http://search.msn.com/results.aspx?q=windows+vista&mkt=en-US&form=QBRE" target="_new">Learn more!</a><div style="margin-top: 0px; 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