<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Dear Chen: just two remarks that may or may not be relevant.<div>1) The "measured" forces seem to me rather small in magnitude. There may be accuracy issues. I would try to increase the value of the electric field and check that estimated effective charges are numerically stable in the range of the linear regime (this check should always be done when estimating derivatives numerically).</div><div>2) I am out of town at this time, without any textbook handy, so I cannot check the correct definitions and current nomenclature. However, in the literature one defines two different "effective charges" which differ by a factor equal to the dielectric constant (basically, one is the force generated by an unit *applied* field, the other is the one generated by a unit *internal* field. You may want to check the consistency of what you calculate with what the phonon code provides.</div><div>Cheers - SB</div><div><br><div><div>On Oct 4, 2008, at 2:10 AM, alan chen wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr"><p>Dear PWSCF users,<br> In order to check whether the forces in the 'sawtooth' method are calculated correctly in the latest version-4.0.2(no offense intended), we carry out the following test. We calculate an isolated symmetric 5-layer STO slab (input file is attached below). In the absence of electric field, there are no forces on the middle TiO2 layer due to symmetry. Then we turn on a weak electric field (0.0001 a.u.) and obtain the forces on Ti and on O are 0.00046 and -0.00013 Ry/au, respectively (in the middle TiO2 layer). Therefore the Born effective charges of Ti and O are approximately </p><p>e*(Ti)=0.00046/0.0001/2= 2.3<br>e*(O)=-0.00013/0.0001/2= -0.65</p><p>The factor '2' is due to the units. However from both phonon frequency calculation and literature, the Born effective charges of Ti and O are: </p><p>e*(Ti)= 7.25<br>e*(O)= -2.06</p><p>I am just wondering whether my method is correct. If it is correct, does that imply there might be something wrong with the forces in the 'sawtooth' method.</p> <div>Thank you very much.</div> <div> </div> <div>Hanghui Chen</div> <div>Department of Physics, Yale University</div> <div> </div> <div>&CONTROL<br> calculation='scf'<br> wf_collect=.true.<br> pseudo_dir = './psp'<br> outdir='.'<br> wfcdir='/scratch'<br> prefix='STO-5.5uc-EF0.0001-scf' <br> tprnfor = .true.<br> tstress = .true.<br> disk_io='low'<br> verbosity='high'<br> dt=80.D0<br> tefield=.true.<br>/<br>&SYSTEM <br> ibrav= 6<br> celldm(1) = 7.27<br> celldm(3) = 10.0<br> nat= 27<br> ntyp= 3<br> ecutwfc = 30.0<br> ecutrho = 180.0<br> occupations='smearing'<br> smearing=gauss<br> degauss=0.005<br> edir=3<br> emaxpos=0.8<br> eopreg=0.1<br> eamp=0.0001<br>/<br>&ELECTRONS<br> diagonalization='david'<br> mixing_beta = 0.7D0<br> diago_david_ndim = 4<br> startingwfc='atomic'<br> startingpot='file'<br> conv_thr = 1.0d-8<br>/<br>&IONS<br> ion_dynamics = 'damp'<br> phase_space = 'full'<br> pot_extrapolation = 'second_order'<br> wfc_extrapolation = 'second_order'<br>/<br>&CELL<br> cell_dynamics = 'damp-w'<br>/<br>ATOMIC_SPECIES<br> Sr 87.62 038-Sr-ca-sp-vgrp.uspp.format.UPF<br> Ti 47.90 022-Ti-ca-sp-vgrp.uspp.format.UPF<br> O 16.00 008-O-ca--vgrp.uspp.format.UPF<br>ATOMIC_POSITIONS alat<br> Sr 0.00 0.00 0.00<br> O 0.50 0.50 0.00<br> Ti 0.50 0.50 0.50<br> O 0.00 0.50 0.50<br> O 0.50 0.00 0.50<br> Sr 0.00 0.00 1.00 <br> O 0.50 0.50 1.00<br> Ti 0.50 0.50 1.50<br> O 0.00 0.50 1.50<br> O 0.50 0.00 1.50<br> Sr 0.00 0.00 2.00 <br> O 0.50 0.50 2.00<br> Ti 0.50 0.50 2.50<br> O 0.00 0.50 2.50<br> O 0.50 0.00 2.50<br> Sr 0.00 0.00 3.00 <br> O 0.50 0.50 3.00<br> Ti 0.50 0.50 3.50<br> O 0.00 0.50 3.50<br> O 0.50 0.00 3.50<br> Sr 0.00 0.00 4.00 <br> O 0.50 0.50 4.00<br> Ti 0.50 0.50 4.50<br> O 0.00 0.50 4.50<br> O 0.50 0.00 4.50<br> Sr 0.00 0.00 5.00 <br> O 0.50 0.50 5.00<br>K_POINTS {automatic}<br>8 8 1 0 0 0<br></div> <div> </div> <div> </div> <div> </div> <div><br> </div></div> _______________________________________________<br>Pw_forum mailing list<br><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>http://www.democritos.it/mailman/listinfo/pw_forum<br></blockquote></div><br><div apple-content-edited="true"> <span class="Apple-style-span" style="border-collapse: separate; 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; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; 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; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; 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; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; 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; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">---</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">Stefano Baroni - SISSA</span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "> </span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">&</span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "> </span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">DEMOCRITOS National Simulation Center - Trieste</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "><a href="http://www.sissa.it/~baroni">http://www.sissa.it/~baroni</a> / [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; min-height: 14px; font-size: 12px; "><br style="font-size: 12px; "></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; min-height: 14px; font-size: 12px; "><span class="Apple-style-span" style="font-size: 14px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "><br></span></font></div></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">Please, if possible, don't</span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "> </span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">send me MS Word or PowerPoint attachments</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; ">Why? See:</span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px; "> </span></font><font class="Apple-style-span" color="#0023E9" size="3"><span class="Apple-style-span" style="font-size: 12px; "><a href="http://www.gnu.org/philosophy/no-word-attachments.html">http://www.gnu.org/philosophy/no-word-attachments.html</a></span></font></div><div><font class="Apple-style-span" color="#0023E9" size="3"><span class="Apple-style-span" style="font-size: 12px; "><br class="webkit-block-placeholder"></span></font></div></div></div></span><br class="Apple-interchange-newline"></div></span><br class="Apple-interchange-newline"></div></span><br class="Apple-interchange-newline"></div></span><br class="Apple-interchange-newline"> </div><br></div></body></html>