<div dir="ltr">Hello Quantum Espresso users,<div><br></div><div>I am trying to use QE to get the real space harmonic force constants using pw.x, ph.x and q2r.x in the same order for Si. I have attached all my input files. I am using a 5X5X5 mesh.</div><div><br></div><div>I used ibrav=0 and my lattice vectors are [0 0.5 0.5], [0.5 0 0.5], [0.5 0.5 0] in alat units. So if I understand the convention given in the faq phonon section of the documentation, the first nearest neighbors should have the indices [1 1 1], [5 1 1], [1 5 1] and [1 1 5].</div><div><br></div><div>From my understanding, the first nearest neighbours for Si have the diagonal term in the force constant matrix +/- alpha and off-diagonal term in the force constant matrix +/- beta for some alpha and beta. </div><div><br></div><div><div style="font-size:12.8000001907349px">Now, the [1 1 1] force constant seems to have the right symmetries :</div><div style="font-size:12.8000001907349px">atom pair : 1 2</div><div style="font-size:12.8000001907349px">148 1 1 1 -6.40122756000E-02 11</div><div style="font-size:12.8000001907349px">652 1 1 1 -4.29620484800E-02 12</div><div style="font-size:12.8000001907349px">1156 1 1 1 -4.29620484800E-02 13</div><div style="font-size:12.8000001907349px">1660 1 1 1 -4.29620484800E-02 21</div><div style="font-size:12.8000001907349px">2164 1 1 1 -6.40122756000E-02 22</div><div style="font-size:12.8000001907349px">2668 1 1 1 -4.29620484800E-02 23</div><div style="font-size:12.8000001907349px">3172 1 1 1 -4.29620484800E-02 31</div><div style="font-size:12.8000001907349px">3676 1 1 1 -4.29620484800E-02 32</div><div style="font-size:12.8000001907349px">4180 1 1 1 -6.40122756000E-02 33</div><div><br></div><div><div style="font-size:12.8000001907349px">However, the [5 1 1] (and [1 5 1], [1 1 5]) force constants do not have the right symmetries </div><div style="font-size:12.8000001907349px">atom pair : 1 2</div><div style="font-size:12.8000001907349px">152 5 1 1 -3.87971851534E-03 11</div><div style="font-size:12.8000001907349px">656 5 1 1 1.05471196450E-03 12</div><div style="font-size:12.8000001907349px">1160 5 1 1 1.05471196450E-03 13</div><div style="font-size:12.8000001907349px">1664 5 1 1 1.05471196450E-03 21</div><div style="font-size:12.8000001907349px">2168 5 1 1 -3.60176689065E-04 22</div><div style="font-size:12.8000001907349px">2672 5 1 1 -3.96403112877E-04 23</div><div style="font-size:12.8000001907349px">3176 5 1 1 1.05471196450E-03 31</div><div style="font-size:12.8000001907349px">3680 5 1 1 -3.96403112877E-04 32</div><div style="font-size:12.8000001907349px">4184 5 1 1 -3.60176689065E-04 33</div><div><br></div><div><b style="font-size:12.8000001907349px">The first column is the line number in my file, then followed by the three indices for the lattice vectors, then the force constant and then finally the component (11=xx, 12=xy and so on)</b><br></div><div><b style="font-size:12.8000001907349px"><br></b></div><div><span style="font-size:12.8000001907349px">Does anyone have an idea why this happens?</span></div><div><span style="font-size:12.8000001907349px"><br></span></div><div><span style="font-size:12.8000001907349px">P.S. : In the <a href="http://si.scf.in">si.scf.in</a> file, I have CELL_PARAMETERS {cubic}. I have also tried with CELL_PARAMETERS {alat} and there is no difference.</span></div><div><span style="font-size:12.8000001907349px"><br></span></div><div><span style="font-size:12.8000001907349px">Thanks,</span></div><div><span style="font-size:12.8000001907349px">Navaneeth</span></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></div></div>