<HTML><BODY>Dear QE developers and users,<br><br>I've calculated phonon dispersion for alpha-quartz. It gives for acoustic branches at Gamma --- A direction:<br><br><p>q (2*pi/a_0) omega_1 omega_2 omega_3 (cm^(-1))<br>------------------------------------------------------------------<br>0.00000000 -1.1614 -0.8602 -0.7608<br>0.045470455 9.2277 18.1443 24.0697<br>0.09094091 21.8192 34.5505 47.4024<br>. . .</p>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.<br><br>My scf input:<br><br><p>&control<br> calculation='scf',<br> restart_mode='from_scratch',<br> prefix='sio2',<br> pseudo_dir = '/home/user/espresso/pseudo',<br> outdir='/home/user/espresso/tmp'<br> /<br> &system<br> ibrav=4,<br> celldm(1)=9.289897331,<br> celldm(3)=1.099552482,<br> nat= 9, ntyp= 2,<br> ecutwfc = 20.0<br> ecutrho = 150.0<br> /<br> &electrons<br> electron_maxstep=1000<br> mixing_mode = 'plain'<br> mixing_beta = 0.7<br> conv_thr = 1.0d-8<br> /<br>ATOMIC_SPECIES<br> Si 28.086 Si.pw91-n-van.UPF<br> O 15.999 O.pw91-van_ak.UPF<br>ATOMIC_POSITIONS crystal<br>Si 0.465976452 0.000000000 0.000000000<br>Si -0.000045477 0.465778133 0.666710600<br>Si -0.465823610 -0.465778133 0.333289400<br>O 0.410406470 0.274336629 0.112688982<br>O -0.274215479 0.135800008 0.779283047<br>O -0.136110820 -0.409868933 0.446516439<br>O 0.273758113 0.409868933 0.553483561<br>O -0.410015489 -0.135800008 0.220716953<br>O 0.136069841 -0.274336629 -0.112688982<br>K_POINTS automatic<br>4 4 4 0 0 0<br><br><br>Phonon input:</p><p>phonons of SiO_2<br> &inputph<br> tr2_ph=1.0d-12,<br> prefix='sio2',<br> ldisp=.true.,<br> nq1=2, nq2=2, nq3=2<br> amass(1)=28.086,<br> amass(2)=15.999,<br> outdir='/home/user/espresso/tmp',<br> fildyn='sio2.dyn',<br> /</p><p><br>Does anyone know the origin of such discrepancy? </p>Regards,<br>Mikhail Goncharovskii,<br>postgraduate student, Omsk State University, Russia<br><br><br>-------------------------------------------------------------------------------------------------------------------<br>Please, if possible, don't send me MS Word or PowerPoint attachments.<br>Why? See: http://www.gnu.org/philosophy/no-word-attachments.html<br></BODY></HTML>