<div dir="ltr"><div><div><div><div><div><div><div><div><div><div><div><div><div><div><div>Dear Stefano,<br><br></div><div>Thanks for your reply. I still have a couple of unresolved issues.<br><br></div>Silicon has an inversion center about (a/8, a/8, a/8) right? So shouldn't the force constants also have an inversion symmetry? i.e., Phi(na=1,nb=1)_xy(R=a1) ==
Phi(nb=1,na=1)_xy(R=-a1) apart from Phi(na=1,nb=1)_yx(R=a1) ==
Phi(nb=1,na=1)_xy(R=-a1). So Phi(na=1,nb=1)_yx(R=a1) ==
Phi(nb=1,na=1)_xy(R=a1)?<br><br></div>I also found another problem with my computations. The force constants matrix between atoms at (0, 0, 0) and (a/4, a/4, a/4) and also the other 3 equivalent points around (0, 0, 0) which make the first nearest neighbour shell should be of the form<br><br></div>Phi_xx = a;<br></div>Phi_xy = +/- b;<br></div>Phi_xz = +/- b;<br><br></div>Phi_yx = +/- b;<br></div>Phi_yy = a;<br></div>Phi_yz = +/- b;<br><br></div>Phi_zx = +/- b;<br></div>Phi_zy = +/- b;<br></div>Phi_zz = a;<br><br></div>right?<br><br></div>In the force constants file I had sent to the forum earlier, this is not the case. In fact, there are 4 matrices of this type but are associated with other atomic positions. I have this problem only with the second atom in the basis. The force constants in connecting the first atoms in the basis Phi(nb=1,na=1) seem to have the expected symmetries. For brevity, I am attaching a force constants file with 3X3X3 grid.<br><br></div>To point out one such example, please have a look at the following section of the force constants file <br>(Format : Line number, tau_1, tau_2, tau_3, Phi_xx(na=1, nb=2) ):<br><br> <b> 47 1 1 1 -6.84986974074E-02</b><br> 48 2 1 1 1.06531878102E-04<br> 49 3 1 1 1.11977775153E-03<br><b> 50 1 2 1 -6.84986969382E-02</b><br> 51 2 2 1 -6.67371571926E-04<br> <b>52 3 2 1 -6.84986979674E-02</b><br> 53 1 3 1 -7.21441565439E-03<br> 54 2 3 1 -6.67372403004E-04<br> 55 3 3 1 1.11977897933E-03<br> 56 1 1 2 1.06531878102E-04<br> 57 2 1 2 6.18314184023E-04<br> 58 3 1 2 1.11977814815E-03<br> 59 1 2 2 -6.67371571926E-04<br> 60 2 2 2 -7.21441521994E-03<br> 61 3 2 2 1.11977888383E-03<br> 62 1 3 2 -6.67372403004E-04<br> 63 2 3 2 6.18311460290E-04<br> 64 3 3 2 6.18312968022E-04<br> 65 1 1 3 1.11977775153E-03<br> 66 2 1 3 1.11977814815E-03<br> 67 3 1 3 6.72231001162E-04<br> <b>68 1 2 3 -6.84986979674E-02</b><br> 69 2 2 3 1.11977888383E-03<br> 70 3 2 3 1.06531502673E-04<br> 71 1 3 3 1.11977897933E-03<br> 72 2 3 3 6.18312968022E-04<br> 73 3 3 3 1.06533738456E-04<br><br></div><div>Line numbers 47, 50, 52 and 68 (highlighted in bold font) seem to have the same value (a). However, the lattice points corresponding to these lines are not at the locations where the first nearest neighbours are located for diamond lattice (Si). From my understanding, these lattice vectors correspond to (0 0 0), (0 a/2 a/2), (-a/2 0 a/2) and (-a/2 a/2 0), but they should have been (0, 0, 0), (-a/2, -a/2, 0), (0, -a/2, -a/2) and (-a/2, 0, a/2). I have checked this with 3X3X3 to 8X8X8 grid, the problem persists.<br><br></div><div>It is quite possible that I have misunderstood the syntax. I would appreciate it if you or someone else in the forum could clarify my questions. Thanks for your time!<br><br></div><div>--Navaneeth<br></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Nov 11, 2014 at 2:53 AM, stefano de gironcoli <span dir="ltr"><<a 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">
<div bgcolor="#FFFFFF" text="#000000">
<div>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.<span class="HOEnZb"><font color="#888888"><br>
<br>
stefano</font></span><div><div class="h5"><br>
<br>
<br>
On 11/11/2014 08:23 AM, Navaneetha Krishnan wrote:<br>
</div></div></div><div><div class="h5">
<blockquote type="cite">
<div dir="ltr">
<div>
<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 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">
<div bgcolor="#FFFFFF" text="#000000">
<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><br>
<br>
<br>
On 11/10/2014 09:16 AM, Navaneetha Krishnan wrote:<br>
</div>
</div>
</div>
<blockquote type="cite">
<div>
<div>
<div dir="ltr">
<div>
<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>
<div dir="ltr">
<div>
<div>Caltech.</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
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<div>Navaneetha Krishnan Ravichandran,
<div>Graduate Student,</div>
<div>Mechanical Engineering,</div>
<div>Caltech.</div>
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