<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>On Jan 8, 2012, at 6:51 PM, Paolo Giannozzi wrote:</div><div><br></div><div>just a few more more words<br><div></div></div><br><blockquote type="cite"><div>On Jan 8, 2012, at 17:43 , Caloma Trumica wrote:<br><br><blockquote type="cite">why do we need to have such an additional setting of q points<br></blockquote><blockquote type="cite">rather than just using k points mesh for both?<br></blockquote><br>because they refer to different quantities and have different<br>convergence criteria. The "k-point" grid is used to perform the<br>integral over the Brillouin Zone; </div></blockquote><div><br></div>... for electronic properties (squared Bloch states -> charge density, 1-electron energies -> sum thereof, etc.)</div><div><br><blockquote type="cite"><div>the "q-point" grid is used to<br>perform integrals over phonon wave vectors, </div></blockquote><div><br></div>... which is also a sum over the Brillouin zone, but of quantities belonging to a different class of properties, which usually display different convergence properties (as pointed out by Paolo)</div><div><br><blockquote type="cite"><div>or to obtain<br>Interatomic Force Constants in real space.<br></div></blockquote><div><br></div>as above ...</div><div><br></div><div>Also, q-point meshes usually include the gamma point (both because technically they often appear as differences between "k-points" q=k2-k1, hence if k2 and k1 both belong to a same mesh, q can be equal to zero; and also because one is often interested in zone-center phonons per se); k-points, instead, are often offset from the origin, because the sum over points of a regular grid of a periodic function converges slightly better as a function of the number of points if the grid is offset (this the somewhat obscure logics behind the construction of the Monkhorst-Pack sets of "special points").</div><div><br></div><div>SB</div><div><br></div><div>
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