[Pw_forum] Too low speed of sound in quartz

[Гриша Гончаровский]

Dear QE developers and users,

I've calculated phonon dispersion for alpha-quartz. It gives for acoustic branches at Gamma --- A direction:

q (2*pi/a_0)       omega_1   omega_2   omega_3 (cm^(-1))
------------------------------------------------------------------
0.00000000       -1.1614      -0.8602       -0.7608
0.045470455      9.2277      18.1443     24.0697
0.09094091       21.8192     34.5505     47.4024
. . .Differentiating these dispersion relations I found that the speed of the "fastest" sound waves is about 1000 m/sec, while the simple mechanical calculation (as well as reference data) gives at least a five times larger value.

My scf input:

&control
 calculation='scf',
 restart_mode='from_scratch',
 prefix='sio2',
 pseudo_dir = '/home/user/espresso/pseudo',
 outdir='/home/user/espresso/tmp'
 /
 &system
 ibrav=4,
 celldm(1)=9.289897331,
 celldm(3)=1.099552482,
 nat= 9, ntyp= 2,
 ecutwfc = 20.0
 ecutrho = 150.0
 /
 &electrons
 electron_maxstep=1000
 mixing_mode = 'plain'
 mixing_beta = 0.7
 conv_thr = 1.0d-8
 /
ATOMIC_SPECIES
 Si 28.086 Si.pw91-n-van.UPF
 O 15.999 O.pw91-van_ak.UPF
ATOMIC_POSITIONS crystal
Si 0.465976452 0.000000000 0.000000000
Si -0.000045477 0.465778133 0.666710600
Si -0.465823610 -0.465778133 0.333289400
O 0.410406470 0.274336629 0.112688982
O -0.274215479 0.135800008 0.779283047
O -0.136110820 -0.409868933 0.446516439
O 0.273758113 0.409868933 0.553483561
O -0.410015489 -0.135800008 0.220716953
O 0.136069841 -0.274336629 -0.112688982
K_POINTS automatic
4 4 4 0 0 0


Phonon input:
phonons of SiO_2
 &inputph
 tr2_ph=1.0d-12,
 prefix='sio2',
 ldisp=.true.,
 nq1=2, nq2=2, nq3=2
 amass(1)=28.086,
 amass(2)=15.999,
 outdir='/home/user/espresso/tmp',
 fildyn='sio2.dyn',
 /

Does anyone know the origin of such discrepancy? Regards,
Mikhail Goncharovskii,
postgraduate student, Omsk State University, Russia


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