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<span style="margin:0px;font-size:12pt">Hello, QE Users,</span>
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<div style="margin:0px;font-size:12pt">I am using Quantum Espresso v6.7 and having some issues with berry phase calculation to find ferroelectric polarization. I cannot find the significance of using mod 2 for ionic polarization calculation. Let's take BaTiO3
as an example.</div>
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<div style="margin:0px;font-size:12pt">For non-centrosymmetric BaTiO3, the ionic polarization reported is as follows:</div>
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<div style="margin:0px;font-size:12pt"><img size="0" contenttype="image/png" unselectable="on" tabindex="-1" style="margin:0px;max-width:100%;user-select:none" data-outlook-trace="F:1|T:1" src="cid:eca35530-51f2-41d6-9f98-c362c7cb49d3"><br>
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<div style="margin:0px;font-size:12pt"><b>Case-I:</b> If I look at the z-coordinate of Ti, it is slightly upward than the centrosymmetric position and the ionic phase is calculated as charge*position*G-vector=12*0.5199*0.98912=6.17=0.17(mod 2)</div>
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<div style="margin:0px;font-size:12pt"><b>Case-II:</b> If Ti shifts more upward, for example z co-ordinate is 0.61, then the ionic phase = 12*0.61*0.98912= 7.24= -0.76 (mod 2)</div>
<div style="margin:0px;font-size:12pt"><span style="margin:0px;background-color:rgb(255, 255, 255);display:inline !important"><b>Case-III:</b> If Ti is in centrosymmetric position, then ionic phase contribution=12*0.5*.98912=5.93= -0.07 (mod 2)</span><br>
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<div style="margin:0px;font-size:12pt">Although in both case-I and case-II, Ti shifts upward, first one gives positive ionic phase contribution while second one gives negative ionic phase contribution with reference to case-III.<span> </span><span style="margin:0px;background-color:rgb(255, 255, 255);display:inline !important">Both
should have given same sign of ionic phase. </span><b>So, why should we take mod 2 wrapping?</b></div>
<div style="margin:0px;font-size:12pt"><span style="margin:0px">May be 0.5 to 0.61 ionic displacement is not possible for BaTiO3, but it is the case for some other ferroelectrics i.e. HfO2. I know that polarization calculation in quantum espresso is not thoroughly
tested. But what should be the general procedure for finding ionic polarization of all ferroelectrics? Your response will highly oblige me.</span></div>
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<div style="margin:0px;font-size:12pt"><span style="margin:0px">I am attaching input files for scf and non-scf calculation herewith.</span></div>
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<div style="margin:0px;font-size:12pt">-Regards</div>
<div style="margin:0px;font-size:12pt">Tanmoy Kumar Paul</div>
<div style="margin:0px;font-size:12pt">1st year Graduate Student</div>
<div style="margin:0px;font-size:12pt">Purdue University, USA</div>
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<div style="margin:0px;font-size:12pt"><u><b>.scf input:</b></u></div>
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</u></div>
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<div style="margin:0px"> calculation = 'scf'</div>
<div style="margin:0px"> restart_mode='from_scratch',</div>
<div style="margin:0px"> prefix='asymm',</div>
<div style="margin:0px"> tstress = .true. </div>
<div style="margin:0px"> tprnfor = .true.</div>
<div style="margin:0px"> pseudo_dir = 'Quantum_espresso/pseudos',</div>
<div style="margin:0px"> outdir='Quantum_espresso/outdirb',</div>
<div style="margin:0px"> /</div>
<div style="margin:0px"> &system</div>
<div style="margin:0px"> ibrav= 6, </div>
<div style="margin:0px"> nat= 5,</div>
<div style="margin:0px"> ntyp= 3,</div>
<div style="margin:0px"> celldm(1)=7.5438,</div>
<div style="margin:0px"> celldm(3)=1.011</div>
<div style="margin:0px"> ecutwfc =60.0,</div>
<div style="margin:0px"> ecutrho=600.0,</div>
<div style="margin:0px"> occupations='fixed',</div>
<div style="margin:0px"> /</div>
<div style="margin:0px"> &electrons</div>
<div style="margin:0px"> mixing_mode = 'plain'</div>
<div style="margin:0px"> mixing_beta = 0.4</div>
<div style="margin:0px"> conv_thr = 1.0d-8</div>
<div style="margin:0px"> /</div>
<div style="margin:0px"> &ions</div>
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<div style="margin:0px"> &cell</div>
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<div style="margin:0px">ATOMIC_SPECIES</div>
<div style="margin:0px">Ba 137.34 Ba.pbe-nsp-van.UPF</div>
<div style="margin:0px">Ti 47.90 Ti.pbe-sp-van_ak.UPF</div>
<div style="margin:0px">O 16.0 O.pbe-rrkjus.UPF</div>
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<div style="margin:0px">ATOMIC_POSITIONS {crystal}</div>
<div style="margin:0px">Ba 0.0000000000 0.0000000000 0</div>
<div style="margin:0px">Ti 0.5000000000 0.5000000000 0.51427</div>
<div style="margin:0px">O 0.5000000000 0.5000000000 0.974477</div>
<div style="margin:0px">O 0.5000000000 0.0000000000 0.4876</div>
<div style="margin:0px">O 0.0000000000 0.5000000000 0.4876</div>
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<div style="margin:0px">K_POINTS {automatic}</div>
5 5 5 0 0 0<br>
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<div style="margin:0px;font-size:12pt"><u><b>.nscf input:</b></u></div>
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</u></div>
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<div style="margin:0px"> calculation = 'nscf'</div>
<div style="margin:0px"> prefix='asymm',</div>
<div style="margin:0px"> pseudo_dir = 'Quantum_espresso/pseudos',</div>
<div style="margin:0px"> outdir='Quantum_espresso/outdirb',</div>
<div style="margin:0px"> lberry=.true. ,</div>
<div style="margin:0px"> gdir = 3,</div>
<div style="margin:0px"> nppstr = 20</div>
<div style="margin:0px"> /</div>
<div style="margin:0px"> &system</div>
<div style="margin:0px"> ibrav= 6, </div>
<div style="margin:0px"> nat= 5,</div>
<div style="margin:0px"> ntyp= 3,</div>
<div style="margin:0px"> celldm(1)=7.5438,</div>
<div style="margin:0px"> celldm(3)=1.011</div>
<div style="margin:0px"> ecutwfc =60.0,</div>
<div style="margin:0px"> ecutrho=600.0,</div>
<div style="margin:0px"> occupations='fixed', </div>
<div style="margin:0px"> /</div>
<div style="margin:0px"> &electrons</div>
<div style="margin:0px"> mixing_mode = 'plain'</div>
<div style="margin:0px"> mixing_beta = 0.4</div>
<div style="margin:0px"> conv_thr = 1.0d-8</div>
<div style="margin:0px"> /</div>
<div style="margin:0px"> &ions</div>
<div style="margin:0px"> /</div>
<div style="margin:0px"> &cell</div>
<div style="margin:0px"> /</div>
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<div style="margin:0px">ATOMIC_SPECIES</div>
<div style="margin:0px">Ba 137.34 Ba.pbe-nsp-van.UPF</div>
<div style="margin:0px">Ti 47.90 Ti.pbe-sp-van_ak.UPF</div>
<div style="margin:0px">O 16.0 O.pbe-rrkjus.UPF</div>
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<div style="margin:0px">ATOMIC_POSITIONS {crystal}</div>
<div style="margin:0px">Ba 0.0000000000 0.0000000000 0</div>
<div style="margin:0px">Ti 0.5000000000 0.5000000000 0.51427</div>
<div style="margin:0px">O 0.5000000000 0.5000000000 0.974477</div>
<div style="margin:0px">O 0.5000000000 0.0000000000 0.4876</div>
<div style="margin:0px">O 0.0000000000 0.5000000000 0.4876</div>
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<div style="margin:0px">K_POINTS {automatic}</div>
5 5 20 0 0 0<br>
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