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<br><div><div>On Oct 24, 2007, at 4:36 AM, Yanyan wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">Dear Stefano de Gironcoli</font></font></span></pre><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000"></font></font></span></pre><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000"> Recently, i am testing the virtual-atom pseudopotential generated by virtual.x.</font></font></span></pre><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">I want to ask if the testing results of the virtual pseudo made by 0% pseudo1 + 100% pseudo2</font></font></span></pre><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">are in same agreement with the results calculated directly with pseudo2. </font></font></span></pre></blockquote><div><br class="webkit-block-placeholder"></div>They should, as far as I understand, but others may have something more to add.</div><div><br><blockquote type="cite"><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">If both </font></font></span></pre><pre><span lang="EN-US" style="COLOR: red;
FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">values are different, only the convergence trend is equal. Is the virtual-pseudo OK? </font></font></span></pre><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">While i want to </font></font></span><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">calculate phonon frequencies, </font></font></span><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">how can I do to obtain the mass </font></font></span></pre><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">of virtual-atom pseudopotential. Because of </font></font></span><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">the mixing pseudopotential as (1-x) </font></font></span></pre><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">pseudopotential1 + x pseudopotential2, </font></font></span><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">whether the mass of virtual-atom </font></font></span></pre><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">pseudopotential can be determined by (1-x)</font></font></span><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000"> mass of potential1 + x mass of potential2.</font></font></span></pre></blockquote><div><br class="webkit-block-placeholder"></div>As already said in this forum a few days ago, it depends on the application. To be (a little bit) more specific: the weighted mass gives reasonable results for acoustic phonons, where atoms of different mass vibrate in phase. For the optic branches, instead, different atoms vibrate out of phase, so that taking the average mass is only meaningful when the optic branches of the pure compounds overlap considerably (which in turn implies that the masses are close enough). Beware, though, that the time-consuming part of any phonon calculation is the computation of the interatomic force constants, which do not depend on mass. Once these calculated, the calculation and diagonalization of the dynamical matrices is a relatively minor task. It may make sense therefore to calculate the interatomic force constants using the VCA, and then to calculate the dynamical matrices using real masses. This procedure has been followed successfully since the eve of DFPT. Examples of its application can be found, e.g., in Rev. Mod. Phys. <font style="font: 12.0px Helvetica"><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;"> </span></font></font><font style="font: 10.0px Helvetica"><font class="Apple-style-span" size="4"><span class="Apple-style-span" style="font-size: 14px;">73, 515 (2001).</span></font></font> </div><div><br class="webkit-block-placeholder"></div><div>Hope this helps.</div><div>Stefano (Baroni)</div><div><br><blockquote type="cite"><pre><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000"><o:p></o:p></font></font></span></pre><pre style="TEXT-INDENT: 12pt"><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">Thank you for your advice.<o:p></o:p></font></font></span></pre><pre style="TEXT-INDENT: 12pt"><span lang="EN-US" style="COLOR: red; FONT-FAMILY: 'Times New Roman'"><font size="3"><font color="#000000">Best regards<o:p></o:p></font></font></span></pre><pre style="TEXT-INDENT: 12pt"><span lang="EN-US" style="COLOR: red; 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