[Pw_forum] Problems with phonon calculation in one-dimensional chains

[Nicola Marzari]

Dear Jin,

One possibility is that the chain wants to dimerize - i.e.
have an alternation of short or long bonds. With your current
input, the symmetry of the system is such that the forces are all
zero (and pwscf actually symmetrizes charge densities and forces
so that they satisfy all the symmetry operations that were found
by the code, so that the forces will actually be 0.0000000 with no 
numerical noise). In this way, you are permanently stuck at a
maximum or saddle point of the potential energy surface, instead of
a minimum.

Try breaking the symmetry by hand (e.g. put the first atom in
0.0 0.0 0.1) and relax the structure first.

If that is the case, this is probably something to be added to
http://www.quantum-espresso.org/wiki/index.php/Frequently_Asked_Questions 
. You could write to bonini at mit.edu, and ask a password for the wiki,
and add it yourself !

				nicola
Jin Zhang wrote:
> Dear all,
> 
> 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,
> ---------------------------------------------------------pw.x----------------------------------------------------------
> Infinite Al Chain NCPP
>    &CONTROL
>       calculation = 'scf',
>       prefix='al',
>       nstep=100,
>       tstress = .TRUE.,
>       tprnfor = .TRUE.,
>       outdir='./tmp/',
>       pseudo_dir = '/home/i/opt/espresso- 3.2/pseudo',
>       wf_collect=.true.,
>    /
>    &SYSTEM
>       ibrav=8,
>       celldm(1)=20.0,
>       celldm(2)=1.0,
>       celldm(3)=0.9,
>       nat=2,
>       ntyp=1,
>       ecutwfc=30.0,
>       occupations='smearing',
>       smearing='cold',
>       degauss=0.05,
>       !nbnd=40,
>       la2F=.true.,
>    /
>    &ELECTRONS
>       electron_maxstep = 100,
>       mixing_beta = 0.5,
>       conv_thr =  1.0d-8,
>    /
>    &IONS
>    /
> ATOMIC_SPECIES
> Al  26.98154  Al.pz-vbc.UPF
> ATOMIC_POSITIONS {crystal}
> Al    0.000    0.0   0.0
> Al    0.000    0.0   0.5
> K_POINTS (automatic)
>   1 1 40  0 0 1
> ------------------------------------------------------------ph.x---------------------------------------------------------------
>  &inputph
>   tr2_ph=1.0d-14,
>   recover = .false.
>   prefix='al',
>   fildvscf='dvscf',
>   alpha_mix=0.5,
>   amass(1)=26.98154,
>   outdir='./tmp',
>   fildyn='dyn',
>   elph=.false.,
>   trans=.true.,
>   ldisp=.true.,
>   nq1 =1, nq2 =1, nq3 = 1,
> /
> -------------------------------------------------------------------------------------------------------------------------------
> 
> The obtained frequency is
> 
> ------------------------------------------------------------------------------------------------------------------------------- 
> 
> 
>      omega( 1) =      -3.982548 [THz] =    -132.844370 [cm-1]
>  (  0.000000  0.000000  0.000000  0.000000  0.711951  0.000000 )
>  (  0.000000  0.000000  0.000000  0.000000 -0.702229  0.000000 )
>      omega( 2) =       0.377569 [THz] =      12.594436 [cm-1]
>  ( -0.762581  0.000000  0.000000  0.000000  0.000000  0.000000 )
>  ( -0.646893  0.000000  0.000000  0.000000  0.000000  0.000000 )
>      omega( 3) =       0.377569 [THz] =      12.594436 [cm-1]
>  (  0.000000  0.000000 -0.762581  0.000000  0.000000  0.000000 )
>  (  0.000000  0.000000 -0.646893  0.000000  0.000000  0.000000 )
>      omega( 4) =       0.525357 [THz] =      17.524138 [cm-1]
>  (  0.000000  0.000000  0.000000  0.000000 -0.702229  0.000000 )
>  (  0.000000  0.000000  0.000000  0.000000 -0.711951  0.000000 )
>      omega( 5) =       1.396045 [THz] =      46.567374 [cm-1]
>  (  0.000000  0.000000 -0.646893  0.000000  0.000000  0.000000 )
>  (  0.000000  0.000000  0.762581  0.000000  0.000000  0.000000 )
>      omega( 6) =       1.396045 [THz] =      46.567374 [cm-1]
>  ( -0.646893  0.000000  0.000000  0.000000   0.000000  0.000000 )
>  (  0.762581  0.000000  0.000000  0.000000  0.000000  0.000000 )
> ----------------------------------------------------------------------------------------------------------------------------- 
> 
> 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?
> 
> Looking forward to your help. Thanks in advance!
> 
> Best wishes,
> Jin Zhang
> -- 
> MPhil
> Dept. of Physics, Peking University
> Beijing, 100871, P.R.China
> Phone: 86-10-62768590


-- 
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Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu