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<p>Dear all!</p>
<p><br></p>
<p>I would like to calculate the sound velocity for graphene. It seems</p>
<p>rather simple but I have encountered a problem which I still can't</p>
<p>solve.</p>
<p>I have calculated the phonon band structure which seems reasonable</p>
<p>and have got the frequencies near gamma point (in cm-1).</p>
<p>Now I should just derivate omega (in s-1 = cm-1*3*10^10) by k and obtain velocity. But the</p>
<p>what units of k should I use?</p>
<p>The k units using in PWSCF is 2*pi/a0 where a0=4.66625 Bohr = 2.13</p>
<p>Ang and it seems correct.</p>
<p><br></p>
<p>The reciprocal lattice vectors are following:</p>
<p><br></p>
<p> b(1) = ( 1.0000 0.5774 0.0000 )</p>
<p> b(2) = ( 0.0000 1.1547 0.0000 )</p>
<p> b(3) = ( 0.0000 0.0000 0.1900 )</p>
<p><br></p>
<p>But when I try to evaluate the velocity I obtain the answer nearly in</p>
<p>six time less then reported early: 2200 m/s and 3400 m/s vs. 12200</p>
<p>m/s and 19500 m/s.</p>
<p>I do some stupid mistake, but where? Does anybody has the experience</p>
<p>in such calculations?</p>
<p><br></p>
<p>Thank you!!</p>
<p><br></p>
<p><span class=rvts6>-- </span></p>
<p><span class=rvts6>Sincerely yours,</span></p>
<p><span class=rvts6>Pavel B Sorokin </span><a class=rvts7 href="mailto:PBSorokin@gmail.com">mailto:PBSorokin@gmail.com</a></p>
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