<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ks_c_5601-1987">
<style type="text/css" style="display:none;"><!-- P {margin-top:0;margin-bottom:0;} --></style>
</head>
<body dir="ltr">
<div id="divtagdefaultwrapper" style="font-size:12pt;color:#000000;font-family:Calibri,Helvetica,sans-serif;" dir="ltr">
<p>Dear all</p>
<p><br>
</p>
<p>I am currently trying to calculate the IR spectrum by taking the FFT of the Autocorrelation function obtained from a CPMD run in cp.x (QE 6.1).</p>
<p><br>
</p>
<p>I recently stumbled across a post by <span style="font-size: 12pt;">Giuseppe Mattioli (<a href="http://qe-forge.org/pipermail/pw_forum/2015-July/107471.html" class="OWAAutoLink" id="LPlnk416349" previewremoved="true">http://qe-forge.org/pipermail/pw_forum/2015-July/107471.html</a>) indicating
that applying the MP correction would allow to calculate the FFT of the AC of the dipole; hence my attempt. I obtain (for CO2 centered inside a large empty box):</span></p>
<p></p>
<blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;">
<p><br>
</p>
<p></p>
<div style="">charge density inside the Wigner-Seitz cell: 16.00000000</div>
<p></p>
<p></p>
<div style=""><br>
</div>
<p></p>
<p></p>
<div style=""> reference position (R0): 12.57468453 15.78158723 13.05433235 bohr</div>
<p></p>
<p></p>
<div style=""><br>
</div>
<p></p>
<p></p>
<div style=""> Dipole moments (with respect to x0):</div>
<p></p>
<p></p>
<div style=""> Elect 0.2108 -0.3623 0.1151 au, 0.5358 -0.9209 0.2925 Debye</div>
<p></p>
<p></p>
<div style=""> Ionic 0.0000 0.0000 0.0000 au, 0.0000 0.0000 0.0000 Debye</div>
<p></p>
<p></p>
<div style=""> Total 0.2108 -0.3623 0.1151 au, 0.5358 -0.9209 0.2925 Debye</div>
<p></p>
<p></p>
<div style=""><br>
</div>
<p></p>
<p></p>
<div style=""> Electrons quadrupole moment -139.68377611 a.u.</div>
<p></p>
<p></p>
<div style=""> Ions quadrupole moment 101.51494350 a.u.</div>
<p></p>
<p></p>
<div style=""> Total quadrupole moment -38.16883261 a.u.</div>
<p></p>
<p></p>
<div style=""><br>
</div>
<p></p>
<p></p>
<div style=""> ********* MAKOV-PAYNE CORRECTION *********</div>
<p></p>
<p></p>
<div style=""><br>
</div>
<p></p>
<p></p>
<div style=""> Makov-Payne correction 0.00000000 a.u. = 0.000 eV (1st order, 1/a0)</div>
<p></p>
<p></p>
<div style=""> 0.00001738 a.u. = 0.000 eV (2nd order, 1/a0^3)</div>
<p></p>
<p></p>
<div style=""> 0.00001738 a.u. = 0.000 eV (total)</div>
<p></p>
<p></p>
<div style=""><br>
</div>
<p></p>
<p></p>
<div style=""> corrected Total energy = -37.17673195 a.u.</div>
<p></p>
</blockquote>
<br>
<p></p>
<p>Now a LAMMPS script (<a href="http://lammps.sandia.gov/threads/msg63251.html" class="OWAAutoLink" id="LPlnk368040" previewremoved="true">http://lammps.sandia.gov/threads/msg63251.html</a>) tells me that I am supposed to do the following:</p>
<br>
<p><span style="font-size: 12pt;">calculate the autocorrelation function of the net dipole moment. Compute</span><span style="font-size: 12pt;"> the Fourier transform of the autocorrelation function to get the spectrum.</span><br>
</p>
<p><br>
</p>
<p>Now I wonder which quantity to "autocorrelate". Is it</p>
<p><br>
</p>
<p>"Total_Dipole(timestep)+R0(timestep)"</p>
<p><br>
</p>
<p>Any help is greatly appreciated, especially I wonder if cppp.x is of any help to me on my journey! Input is below (for the ion dynamics)</p>
<p><br>
</p>
<p>Best,</p>
<p>Chris</p>
<p><br>
</p>
<p>Christoph Wolf, Seoul National University, Dept. of Materials Science and Engineering</p>
<p><br>
</p>
<p></p>
<div> &control</div>
<div> calculation = 'cp'</div>
<div> title='CO2 MD'</div>
<div> prefix='co2',</div>
<div> pseudo_dir = '/home/chris/quantum_espresso/pseudo/',</div>
<div> outdir='./'</div>
<div> restart_mode='reset_counters',</div>
<div> nstep=8000, iprint=10, isave=10,</div>
<div> dt=5,</div>
<div> ndr=50, ndw=51</div>
<div> /</div>
<div>&SYSTEM</div>
<div> ibrav = 1</div>
<div> A=15 !relaxed cell value! 5.91 A</div>
<div> nat = 3</div>
<div> ntyp = 2</div>
<div> ecutwfc =80.0</div>
<div> assume_isolated='mp'</div>
<div>/</div>
<div>&electrons</div>
<div> electron_dynamics = 'verlet' </div>
<div> emass=700</div>
<div>/</div>
<div> &ions</div>
<div> ion_dynamics = 'verlet' </div>
<div> ion_temperature='nose' </div>
<div> fnosep=70.0</div>
<div> tempw=300.0</div>
<div>/</div>
<div>ATOMIC_SPECIES</div>
<div> C 12.0107 C_ONCV_PBE-Chris.UPF</div>
<div> O 15.999 O_ONCV_PBE-Chris.UPF</div>
<div>ATOMIC_POSITIONS angstrom</div>
<div> C 7.19147 7.73724 7.21980</div>
<div> O 6.24948 7.68725 5.68537</div>
<div> O 6.69993 9.42553 7.92219</div>
<br>
<p></p>
</div>
</body>
</html>