<p dir="ltr">Dear Giovanni,<br>
Thank you so much for your explanatory response. So, as I understood, there is no way to calculate band structure for a molecule and this is nonsensical. Is it true?</p>
<div class="gmail_extra"><br><div class="gmail_quote">On Nov 29, 2016 1:46 AM, <<a href="mailto:meisam.a63@gmail.com">meisam.a63@gmail.com</a>> wrote:<br type="attribution"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="auto"><div>Just to add that plain wave is not a good approximation to reproduce molecular orbitals.<br><br><div>Mortaza A</div><div><br></div><div>PhD of Physics </div><div>Ulm University, Germany </div></div><div><br>On 28 Nov 2016, at 21:43, Giovanni Cantele <<a href="mailto:giovanni.cantele@spin.cnr.it" target="_blank">giovanni.cantele@spin.cnr.it</a>> wrote:<br><br></div><blockquote type="cite"><div>if you try to ask the question: "what the k-point mesh is for?" you can easily understand<div>that a k-point mesh is for the Brillouin zone (BZ) sampling, that is, perform sums over k-points</div><div>that approximate, with a required level of accuracy, the integral over the whole BZ. This is</div><div>because in a crystal, the energy E(K) depends on the k vectors, and the same holds for the</div><div>wave functions. However, when you build for example a cubic supercell including vacuum to</div><div>simulate a molecule, you are actually dealing with a fictitious cubic crystal where the repeated</div><div>units are sufficiently far apart from each other to not interact.</div><div><br></div><div>Absence of interaction means flat (atomic-like) energy levels, that do not have dependence</div><div>on the k vector. That is, the E(K) dispersion that you get in the case of a molecule, provided</div><div>you include a sufficient vacuum space in your supercell, does not depend on k. If this is</div><div>the case, you can use only the gamma point (a larger grid means large but useless</div><div>computational time, because you are summing over eigenvalues at different k-points that </div><div>are all the same!) for self consistency and BZ path is not needed, because if you select any</div><div>path you should get flat, k-independent bands</div><div><br></div><div>Giovanni</div><div><div><br></div><div><br><div><blockquote type="cite"><div>On 28 Nov 2016, at 18:12, ashkan shekaari <<a href="mailto:shekaari@gmail.com" target="_blank">shekaari@gmail.com</a>> wrote:</div><br class="m_-5599770475287007092Apple-interchange-newline"><div><div dir="ltr"><div><div>Dear experts,<br><br></div>Is it necessary using kpoint mesh when we are dealing with molecules?<br><br></div>In the case of a molecule in cubic vacuum, what path in BZ should be selected?<br clear="all"><div><div><div><div><div><div class="m_-5599770475287007092gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div><span><span style="background-color:rgb(255,255,255)"><b>--</b></span></span></div><div><span><span style="background-color:rgb(255,255,255)"><i style="font-size:12.8px">Regards,</i></span></span></div><div><span><span style="background-color:rgb(255,255,255)"><i>Ashkan Shekaari</i></span></span></div><div><span><span style="background-color:rgb(255,255,255)"><i>Plasma Physics Research Center</i></span></span></div><div><span><span style="background-color:rgb(255,255,255)"><i style="font-size:12.8px"><span style="font-size:12.8px">Science and Research Branch</span></i></span></span></div><div><span><span style="background-color:rgb(255,255,255)"><i>I A U, <span style="font-size:12.8px">14778-93855 Tehran, Iran.</span></i></span></span></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div>
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