Dear all,<br><br>I have encountered several issues related to Gamma point phonon calculation in simple 1d chains(including aluminum, gold and platinum). The symptom is *sometimes* there are one or more eigen-*optical*-modes having negative(usually large) frequencies. The system I considered are all 2-atom/cell 1d chains(to produce
optical branches), with large vacuum region around them(usually
16-20Bohrs). For example, the aluminum input are listed below, <br>---------------------------------------------------------pw.x----------------------------------------------------------<br>Infinite Al Chain NCPP<br> &CONTROL
<br> calculation = 'scf',<br> prefix='al',<br> nstep=100,<br> tstress = .TRUE.,<br> tprnfor = .TRUE.,<br> outdir='./tmp/',<br> pseudo_dir = '/home/i/opt/espresso-
3.2/pseudo',<br> wf_collect=.true.,<br> /<br> &SYSTEM<br> ibrav=8, <br> celldm(1)=20.0, <br> celldm(2)=1.0,<br> celldm(3)=0.9,<br> nat=2, <br> ntyp=1,<br> ecutwfc=30.0,
<br>
occupations='smearing',<br> smearing='cold',<br> degauss=0.05,<br> !nbnd=40,<br> la2F=.true.,<br> /<br> &ELECTRONS<br> electron_maxstep = 100,<br> mixing_beta =
0.5,<br> conv_thr = 1.0d-8,<br> /<br> &IONS<br> /<br>ATOMIC_SPECIES<br>Al 26.98154 Al.pz-vbc.UPF<br>ATOMIC_POSITIONS {crystal}<br>Al 0.000 0.0 0.0<br>Al 0.000 0.0 0.5<br>K_POINTS (automatic)
<br> 1 1 40 0 0 1<br>------------------------------------------------------------ph.x---------------------------------------------------------------<br> &inputph<br> tr2_ph=1.0d-14,<br> recover = .false.<br> prefix='al',
<br> fildvscf='dvscf',<br> alpha_mix=0.5,<br> amass(1)=26.98154,<br> outdir='./tmp',<br> fildyn='dyn',<br> elph=.false.,<br> trans=.true.,<br> ldisp=.true.,<br> nq1 =1, nq2 =1, nq3 = 1,<br>
/<br>-------------------------------------------------------------------------------------------------------------------------------<br><br>The obtained frequency is<br><br>-------------------------------------------------------------------------------------------------------------------------------
<br><br> omega( 1) = -3.982548 [THz] = -132.844370 [cm-1]<br> ( 0.000000 0.000000 0.000000 0.000000 0.711951 0.000000 ) <br> ( 0.000000 0.000000 0.000000 0.000000 -0.702229 0.000000 ) <br> omega( 2) =
0.377569 [THz] = 12.594436 [cm-1]<br> ( -0.762581 0.000000 0.000000 0.000000 0.000000 0.000000 ) <br> ( -0.646893 0.000000 0.000000 0.000000 0.000000 0.000000 ) <br> omega( 3) = 0.377569 [THz] =
12.594436 [cm-1]<br> ( 0.000000 0.000000 -0.762581 0.000000 0.000000 0.000000 ) <br> ( 0.000000 0.000000 -0.646893 0.000000 0.000000 0.000000 ) <br> omega( 4) = 0.525357 [THz] = 17.524138 [cm-1]<br>
( 0.000000 0.000000 0.000000 0.000000 -0.702229 0.000000 ) <br> ( 0.000000 0.000000 0.000000 0.000000 -0.711951 0.000000 ) <br> omega( 5) = 1.396045 [THz] = 46.567374 [cm-1]<br> ( 0.000000 0.000000
-0.646893 0.000000 0.000000 0.000000 ) <br> ( 0.000000 0.000000 0.762581 0.000000 0.000000 0.000000 ) <br> omega( 6) = 1.396045 [THz] = 46.567374 [cm-1]<br> ( -0.646893 0.000000 0.000000 0.000000
0.000000 0.000000 ) <br> ( 0.762581 0.000000 0.000000 0.000000 0.000000 0.000000 ) <br>-----------------------------------------------------------------------------------------------------------------------------
<br>As you can see, the LO mode is largely negative while other modes(including acoustic ones) seem fine. I have made convergence test to k-points, ecut and degauss. In other cases like the gold and platinum chains, the negative modes are usually TO ones. So my question is: Why does this happen? Is there any physics I missed in the above calculation, or just some naive settings make the result weird?
<br><br>Looking forward to your help. Thanks in advance!<br><br>Best wishes,<br>Jin Zhang<br>-- <br>MPhil<br>Dept. of Physics, Peking University<br>Beijing, 100871, P.R.China<br>Phone: 86-10-62768590