<div dir="ltr">Thanks Paolo,<div><br></div><div>Do you think this convention is uniformly applied to any lattice with ibrav=0? (i.e., is there a general form of the convention for lattices with more than two atoms in the basis for eg.,)</div><div><br></div><div>Thanks again,</div><div>-Navaneeth</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Feb 19, 2015 at 3:35 AM, Paolo Giannozzi <span dir="ltr"><<a href="mailto:paolo.giannozzi@uniud.it" target="_blank">paolo.giannozzi@uniud.it</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="">On Mon, 2015-02-09 at 15:45 -0800, Navaneetha Krishnan wrote:<br>
<br>
> I used ibrav=0 and my lattice vectors are [0 0.5 0.5], [0.5 0 0.5],<br>
> [0.5 0.5 0] in alat units. So if I understand the convention given in<br>
> the faq phonon section of the documentation, the first nearest<br>
> neighbors should have the indices [1 1 1], [5 1 1], [1 5 1] and [1 1<br>
> 5].<br>
<br>
</span>The first nearest neighbors of atom 1 = (0,0,0) in cell at (0,0,0) are:<br>
1.) atom 2 = (0.25, 0.25, 0.25) in cell at ( 0.0, 0.0, 0.0);<br>
2.) atom 2 = (0.25, 0.25, 0.25) in cell at ( 0.0,-0.5,-0.5);<br>
3.) atom 2 = (0.25, 0.25, 0.25) in cell at (-0.5, 0.0,-0.5);<br>
4.) atom 2 = (0.25, 0.25, 0.25) in cell at (-0.5,-0.5, 0.0);<br>
<br>
You assume that the indices <=> lattice vector correspondence is:<br>
[1 1 1] == (0, 0, 0), i.e. the two atoms are in the same cell<br>
[2 1 1] == (0, 0.5, 0.5)<br>
[5 1 1] == (0,-0.5,-0.5) (i.e. as above, refolded)<br>
etc, yielding atoms 1 2 + [1 1 1], [5 1 1], [1 5 1], [1 1 5] as<br>
neighbors. This makes sense (I would have made the same assumption),<br>
but apparently the code uses a different sign convention and thinks<br>
that the neighbors are either:<br>
atoms 1 2 + [1 1 1], [2 1 1], [1 2 1], [1 1 2]<br>
or<br>
atoms 2 1 + [1 1 1], [5 1 1], [1 5 1], [1 1 5]<br>
You may verify that these force constants have a more symmetric look<br>
(I did).<br>
<br>
I am not sure where such sign convention comes from, but I am quite<br>
sure that the interatomic force constants are used in a way that is<br>
consistent with the convention adopted for writing them and that<br>
results are correct.<br>
<br>
Paolo<br>
<div><div class="h5"><br>
> From my understanding, the first nearest neighbours for Si have the<br>
> diagonal term in the force constant matrix +/- alpha and off-diagonal<br>
> term in the force constant matrix +/- beta for some alpha and beta.<br>
><br>
><br>
> Now, the [1 1 1] force constant seems to have the right symmetries :<br>
> atom pair : 1 2<br>
> 148 1 1 1 -6.40122756000E-02 11<br>
> 652 1 1 1 -4.29620484800E-02 12<br>
> 1156 1 1 1 -4.29620484800E-02 13<br>
> 1660 1 1 1 -4.29620484800E-02 21<br>
> 2164 1 1 1 -6.40122756000E-02 22<br>
> 2668 1 1 1 -4.29620484800E-02 23<br>
> 3172 1 1 1 -4.29620484800E-02 31<br>
> 3676 1 1 1 -4.29620484800E-02 32<br>
> 4180 1 1 1 -6.40122756000E-02 33<br>
><br>
><br>
> However, the [5 1 1] (and [1 5 1], [1 1 5]) force constants do not<br>
> have the right symmetries<br>
> atom pair : 1 2<br>
> 152 5 1 1 -3.87971851534E-03 11<br>
> 656 5 1 1 1.05471196450E-03 12<br>
> 1160 5 1 1 1.05471196450E-03 13<br>
> 1664 5 1 1 1.05471196450E-03 21<br>
> 2168 5 1 1 -3.60176689065E-04 22<br>
> 2672 5 1 1 -3.96403112877E-04 23<br>
> 3176 5 1 1 1.05471196450E-03 31<br>
> 3680 5 1 1 -3.96403112877E-04 32<br>
> 4184 5 1 1 -3.60176689065E-04 33<br>
><br>
><br>
> The first column is the line number in my file, then followed by the<br>
> three indices for the lattice vectors, then the force constant and<br>
> then finally the component (11=xx, 12=xy and so on)<br>
><br>
><br>
><br>
> Does anyone have an idea why this happens?<br>
><br>
><br>
> P.S. : In the <a href="http://si.scf.in" target="_blank">si.scf.in</a> file, I have CELL_PARAMETERS {cubic}. I have<br>
> also tried with CELL_PARAMETERS {alat} and there is no difference.<br>
><br>
><br>
> Thanks,<br>
> Navaneeth<br>
> --<br>
> Navaneetha Krishnan Ravichandran,<br>
> Graduate Student,<br>
> Mechanical Engineering,<br>
> Caltech.<br>
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</blockquote></div><br><br clear="all"><div><br></div>-- <br><div class="gmail_signature"><div dir="ltr"><div>Navaneetha Krishnan Ravichandran,<div>Graduate Student,</div><div>Mechanical Engineering,</div><div>Caltech.</div></div></div></div>
</div>