<div dir="ltr">Thank you! It makes much more sense now. </div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, May 28, 2013 at 1:47 AM, Axel Kohlmeyer <span dir="ltr"><<a href="mailto:akohlmey@gmail.com" target="_blank">akohlmey@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="im">On Tue, May 28, 2013 at 9:57 AM, Yantao Wu <<a href="mailto:ywu@g.hmc.edu">ywu@g.hmc.edu</a>> wrote:<br>
</div><div><div class="h5">> Dear QE,<br>
><br>
> I'm wondering what ibrav means in pw.x. My confusion is that if you can<br>
> specify atomic positions freely, isn't the structure of the solid already<br>
> implied by the positions? So why do we need the extra variable of ibrav. Or<br>
> does ibrav carries more meaning than just the initial lattice structure of<br>
> the solid?<br>
><br>
> Another related question: When I ran vc-relax, I specified ibrav = 2 (fcc)<br>
> and the atomic positions as follows:<br>
><br>
> ATOMIC_POSITIONS {alat}<br>
> Al 0.0 0.5 0.5<br>
> Al 0.5 0.0 0.0<br>
> Al 0.25 0.25 0.25<br>
> Al 0.0 0.5 0.0<br>
> Ti 0.75 0.75 0.75<br>
> Ti 0.5 0.0 0.5<br>
> Cr 0.0 0.0 0.0<br>
> Cr 0.75 0.75 0.25<br>
> Fe 0.5 0.5 0.0<br>
> Fe 0.75 0.25 0.25<br>
> Co 0.0 0.0 0.5<br>
> Co 0.25 0.75 0.25<br>
> Co 0.75 0.25 0.75<br>
> Ni 0.25 0.75 0.75<br>
> Ni 0.25 0.25 0.75<br>
> Ni 0.5 0.5 0.5<br>
><br>
> QE erred and complained that atom #1 and atom #6 overlap, which they<br>
> apparently don't from the first sight. When I set ibrav = 1 (sc) and kept<br>
> the same atomic positions, the complaint disappeared. So does anyone know<br>
> why did the error occur?<br>
<br>
</div></div>because of the difference between the primitive cell and the<br>
conventional cell. sc and fcc have the same conventional cell, but the<br>
primitive fcc cell contains only a quarter of the atoms. obviously you<br>
entered atoms in the conventional cell.<br>
to be more explicit. the primitive cell of, say, an fcc metal contains<br>
just one atom (one corner) and the basis vectors are not orthogonal,<br>
whereas in the conventional cell, the basis vectors are orthogonal and<br>
you have 4 atoms (one corner and 3 faces).<br>
<br>
Q-E expects that you enter coordinates in the primitive cell only.<br>
yet, since the sc primitive cell is identical to its conventional<br>
cell, you can use an sc lattice to enter an fcc lattice geometry in<br>
the conventional cell.<br>
<br>
look it up in a crystallography text book (and on the web) and you'll see.<br>
<br>
axel.<br>
<div class="im"><br>
><br>
> I appreciate your replies with all my gratitude.<br>
><br>
> Yantao "ignorant student" Wu<br>
><br>
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<br>
<br>
<br>
--<br>
</div>Dr. Axel Kohlmeyer <a href="mailto:akohlmey@gmail.com">akohlmey@gmail.com</a> <a href="http://goo.gl/1wk0" target="_blank">http://goo.gl/1wk0</a><br>
International Centre for Theoretical Physics, Trieste. Italy.<br>
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