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<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-weight: normal; orphans: auto; widows: auto; display: inline !important">I am using pw.x -> ph.x -> dynmat.x to get the LO and TO phonon frequencies at gamma for rock-salt
CaO. The value for the LO phonon I am getting differs from experiment and other DFPT calculations in the literature by 5 THz (167 cm^-1). Perhaps this is a problem with Quantum Espresso, but it’s very possible I’m doing something wrong. I’ve included results
and input files below, and any advice would be greatly appreciated.</span>
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<div>The TO and LO frequencies in THz are given below. Notice that Quantum Espresso using DFPT gives a value that is too low.</div>
<div>experiment: 8.9, 17.1</div>
<div>frozen phonon (Phonopy + Quantum Espresso): 8.71, 17.1</div>
<div>DFPT (Quantum Espresso, pw.x->ph.x->dynmat.x): 8.53, <b>12.2</b></div>
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If I use Phonopy (frozen phonon technique) combined with Quantum Espresso (pw.x and ph.x) I get the correct splitting at gamma. I have tried the same calculation with MgO and I see the same problem, where the frozen phonon technique works while DFPT does not.
Here are some things I have tried: different PAW pseudopotentials, increasing ecutwfc and ecutrho, increasing k-points, reducing conv_thr, and reducing tr2_ph.</div>
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Thanks in advance,</div>
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Casey Brock</div>
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input to pw.x</div>
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&control<br>
calculation='scf'<br>
restart_mode='from_scratch'<br>
prefix='cao'<br>
pseudo_dir = './'<br>
outdir='./output/'<br>
tprnfor = .true.<br>
tstress = .true.<br>
/<br>
&system<br>
ibrav = 0<br>
nat = 2<br>
ntyp = 2<br>
ecutwfc = 70.0<br>
ecutrho = 280.0<br>
/<br>
&electrons<br>
conv_thr = 1.0d-10<br>
mixing_beta = 0.7<br>
/<br>
K_POINTS {automatic}<br>
12 12 12 0 0 0<br>
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CELL_PARAMETERS bohr<br>
4.45 4.45 0<br>
0 4.45 4.45<br>
4.45 0 4.45<br>
ATOMIC_SPECIES<br>
O 16.00 O.pz-n-kjpaw_psl.0.1.UPF<br>
Ca 40.08 Ca.pz-spn-kjpaw_psl.1.0.0.UPF<br>
ATOMIC_POSITIONS crystal<br>
Ca 0.0 0.0 0.0<br>
O 0.5 0.5 0.5</div>
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input to ph.x</div>
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Phonon for CaO<br>
&inputph<br>
prefix='cao',<br>
outdir='./output/',<br>
fildyn='dyn.G'<br>
epsil = .true.<br>
tr2_ph=1.0d-14,<br>
/<br>
0 0 0</div>
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input to dynmat.x<br>
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&input<br>
fildyn = 'dyn.G'<br>
asr = 'simple'<br>
q(1)=0.0, q(2)=0.0, q(3)=1.0<br>
/</div>
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