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Hello QE users,</div>
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I am trying to calculate the phonon properties of different point defects in monolayer (2D) hBN. As a test calculation, I am considering the neutral VnNb defect (nitrogen vacancy with a neighboring boron replaced by a nitrogen). For testing purposes, I am considering
a small 17-atom unit cell with 15 A of vacuum.</div>
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In order to obtain a relaxed structure, it appears that I cannot enforce cell_dofree='ibrav+2Dxy' and instead use cell_dofree='2Dxy'. The resulting forces acting on each atom are zero up to and including the sixth decimal plane. The pw.x output states that
the stresses are 0.00000000 Ry/bhor^3 for each component, expect for the zz component (0.00000086 Ry/bohr^3), which is treated using assume_isolated='2D'.</div>
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My issue is that ph.x calculates negative frequencies for the first two modes (-50 cm^{-1} and -12 cm^{-1}). It doesn't appear to be an issue related to negative frequencies at the Gamma point, because these two modes have very little dispersion across the
Brillouin zone. I have performed a similar calculation for pristine hBN (also using ibrav=0 to verify this isn't the issue) and obtain all positive frequencies. Am I treating the 2D slab correctly? Any thoughts on why the phonon code indicates there is instability?</div>
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<b>pw.x input</b>:</div>
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<span style="font-size: 9pt;">&CONTROL</span>
<div><span style="font-size: 9pt;">calculation = 'scf'</span></div>
<div><span style="font-size: 9pt;">restart_mode = 'from_scratch'</span></div>
<div><span style="font-size: 9pt;">prefix = 'hBN'</span></div>
<div><span style="font-size: 9pt;">pseudo_dir = './pseudo/'</span></div>
<div><span style="font-size: 9pt;">outdir = './TMP_DIR/'</span></div>
<div><span style="font-size: 9pt;">etot_conv_thr = 1.0E-12</span></div>
<div><span style="font-size: 9pt;">tstress = .true.</span></div>
<div><span style="font-size: 9pt;">tprnfor = .true.</span></div>
<div><span style="font-size: 9pt;">/</span></div>
<div><span style="font-size: 9pt;">&SYSTEM</span></div>
<div><span style="font-size: 9pt;">ibrav = 0,</span></div>
<div><span style="font-size: 9pt;">nat = 17,</span></div>
<div><span style="font-size: 9pt;">ntyp = 2,</span></div>
<div><span style="font-size: 9pt;">ecutwfc = 50.0</span></div>
<div><span style="font-size: 9pt;">ecutrho = 600.0</span></div>
<div><span style="font-size: 9pt;">occupations = 'smearing'</span></div>
<div><span style="font-size: 9pt;">smearing = 'm-p'</span></div>
<div><span style="font-size: 9pt;">degauss = 0.0002</span></div>
<div><span style="font-size: 9pt;">assume_isolated = '2D'</span></div>
<div><span style="font-size: 9pt;">nosym = .true. </span></div>
<div><span style="font-size: 9pt;">/</span></div>
<div><span style="font-size: 9pt;">&ELECTRONS</span></div>
<div><span style="font-size: 9pt;">diagonalization = 'cg'</span></div>
<div><span style="font-size: 9pt;">conv_thr = 1.0E-14 </span></div>
<div><span style="font-size: 9pt;">/</span></div>
<div><span style="font-size: 9pt;">&IONS</span></div>
<div><span style="font-size: 9pt;">ion_dynamics="bfgs"</span></div>
<div><span style="font-size: 9pt;">/</span></div>
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<div><span style="font-size: 9pt;">ATOMIC_SPECIES</span></div>
<div><span style="font-size: 9pt;"> B 10.811 B.pbe-n-kjpaw_psl.1.0.0.UPF</span></div>
<div><span style="font-size: 9pt;"> N 14.0067 N.pbe-n-kjpaw_psl.1.0.0.UPF</span></div>
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<div><span style="font-size: 9pt;">ATOMIC_POSITIONS (crystal)</span></div>
<div><span style="font-size: 9pt;">B 0.1236294548 0.2288517946 0.5000000000</span></div>
<div><span style="font-size: 9pt;">B 0.1088156910 0.5613303716 0.5000000000</span></div>
<div><span style="font-size: 9pt;">B 0.1072329417 0.8927673484 0.5000000000</span></div>
<div><span style="font-size: 9pt;">B 0.4864819943 0.2526124675 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.4361682614 0.5638317166 0.5000000000</span></div>
<div><span style="font-size: 9pt;">B 0.4386698609 0.8911844944 0.5000000000</span></div>
<div><span style="font-size: 9pt;">B 0.7855708745 0.2144293085 0.5000000000</span></div>
<div><span style="font-size: 9pt;">B 0.7473878305 0.5135182681 0.5000000000</span></div>
<div><span style="font-size: 9pt;">B 0.7711482933 0.8763705944 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.2442446869 0.1315370037 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.2243740862 0.4579580637 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.2160185581 0.7839816944 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.5616626235 0.1119836884 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.5420420278 0.7756259377 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.8946390297 0.1053610363 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.8880165315 0.4383375889 0.5000000000</span></div>
<div><span style="font-size: 9pt;">N 0.8684632701 0.7557555456 0.5000000000</span></div>
<div><span style="font-size: 9pt;"> </span></div>
<div><span style="font-size: 9pt;">K_POINTS automatic </span></div>
<div><span style="font-size: 9pt;"> 2 2 2 0 0 0 </span></div>
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<div><span style="font-size: 9pt;">CELL_PARAMETERS angstrom</span></div>
<div><span style="font-size: 9pt;"> 7.591938503 -0.231222917 0.000000000</span></div>
<div><span style="font-size: 9pt;"> -3.995637427 6.459556319 0.000000000</span></div>
<span style="font-size: 9pt;"> 0.000000000 0.000000000 15.000000000</span><br>
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<b>ph.x input</b>:</div>
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<span style="font-size: 9pt;">Phonon dispersions for hBN</span>
<div><span style="font-size: 9pt;">&inputph</span></div>
<div><span style="font-size: 9pt;"> prefix = 'hBN'</span></div>
<div><span style="font-size: 9pt;"> outdir = './TMP_DIR/1',</span></div>
<div><span style="font-size: 9pt;"> fildvscf = 'dvscf'</span></div>
<div><span style="font-size: 9pt;"> fildyn = 'hBN.dyn'</span></div>
<div><span style="font-size: 9pt;"> tr2_ph = 1.0d-16</span></div>
<div><span style="font-size: 9pt;"> recover=.true.,</span></div>
<div><span style="font-size: 9pt;"> alpha_mix(1) = 0.1</span></div>
<div><span style="font-size: 9pt;"> search_sym = .true.</span></div>
<div><span style="font-size: 9pt;"> amass(1)=10.811,</span></div>
<div><span style="font-size: 9pt;"> amass(2)=14.0067, </span></div>
<div><span style="font-size: 9pt;"> ldisp=.true.,</span></div>
<div><span style="font-size: 9pt;"> nq1=2, nq2=2, nq3=2,</span></div>
<div><span style="font-size: 9pt;"> start_q=1, last_q=1</span></div>
<span style="font-size: 9pt;">/</span><br>
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Thanks,</div>
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Ken</div>
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