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<div class="moz-cite-prefix">dear Navaneeth,<br>
as noted previously IFC should be symmetric w.r.t exchange of
the two atoms. this include the vector R that joins them.<br>
in your case IFC Phi(na=1,nb=1)_yx(R=a1) ==
Phi(nb=1,na=1)_xy(R=-a1) because when you exchange the two atoms
you need to invert the connecting vector as well.<br>
indeed line 280 is equal to line 802 and line 282 is equal to
800 as they should.<br>
<br>
stefano<br>
<br>
<br>
On 11/11/2014 08:23 AM, Navaneetha Krishnan wrote:<br>
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cite="mid:CACMyjxS506rx5WhHDQOKJWvWAHK5SQcy9x7NZi9WyDPcXW-Hkw@mail.gmail.com"
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<div>Hi Stefano,<br>
<br>
Thanks for your reply. I am particularly looking to address
an anomaly in my simulation. I've attached my scf input, ph
input and q2r input files along with the generated force
constant file.<br>
<br>
</div>
The force constants are not symmetric (in fact anti-symmetric,
for certain direction pairs) with respect to the displacement
directions. For eg., if you look at line number 280 and line
800, the fcs are supposed to be equal as they are Phi_{XY} and
Phi_{YX} for the same pair of atoms. I have tried this with
several combinations of k-sampling and Q-sampling and I get
the same anti-symetry (except when no. of Q's is very small,
like Q=2). I also used a large ecutwfc so that there is no
convergence issue. This problem was not there with Al lattice.
The force constants were symmetric in this sense. Is there bug
in the code or am I making a mistake in one of the input
files? These are not negligible IFCs. Please have a look.<br>
<br>
</div>
<div>Thanks in advance!<br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Nov 10, 2014 at 2:44 AM,
stefano de gironcoli <span dir="ltr"><<a
moz-do-not-send="true" href="mailto:degironc@sissa.it"
target="_blank">degironc@sissa.it</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div>the IFC should be symmetric w.r.t. exchange of the
two atoms and displacement directions as a consequence
of being second order derivatives. other symmetries may
apply in specific cases.<br>
difficult to comment more without more info on what you
are looking for.<br>
stefano
<div>
<div class="h5"><br>
<br>
<br>
On 11/10/2014 09:16 AM, Navaneetha Krishnan wrote:<br>
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<div>Hello QE users,<br>
<br>
</div>
I was investigating the IFCs generated by q2r.x
after scf and ph calculations. It looks like the
force constant matrix for certain pairs of atoms
is not symmetric (with respect to the cartesian
coordinates). For silicon, the IFC matrices have
to be symmetric with respect to the cartesian
coordinates - right? For aluminum, the output is
symmetric, as expected. Any thoughts?<br>
</div>
<div>
<div>
<div><br>
</div>
<div>Thanks a lot!<br>
</div>
<div>--Navaneeth
<div>
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<div>
<div>Caltech.</div>
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-- <br>
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<div>Navaneetha Krishnan Ravichandran,
<div>Graduate Student,</div>
<div>Mechanical Engineering,</div>
<div>Caltech.</div>
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