[Pw_forum] why the phonon eigendisplacement are so much different?

[Yi Kong]

Dear all,
     I want to know the atomic displacement and vibrational amplitude under
different pressures in alloys, for example AB3. At the gamma point, the file
AB3.dynG contains the calculated results, part of the acoustic branches are
shown as following, and I know from the mailing list that after the omega
line is the eigendisplacement of each atom.

**********************************AB3.dynG*********************
.............
     omega( 1) =       1.321104 [THz] =      44.067563 [cm-1]
 (  0.054100  0.000000  0.104646  0.000000 -0.485673  0.000000 )
 (  0.054107  0.000000  0.104658  0.000000 -0.486512  0.000000 )
 (  0.054107  0.000000  0.104827  0.000000 -0.485728  0.000000 )
 (  0.054194  0.000000  0.104658  0.000000 -0.485728  0.000000 )
     omega( 2) =       1.321104 [THz] =      44.067563 [cm-1]
 ( -0.230668  0.000000 -0.427404  0.000000 -0.117786  0.000000 )
 ( -0.230694  0.000000 -0.427453  0.000000 -0.117990  0.000000 )
 ( -0.230694  0.000000 -0.428142  0.000000 -0.117800  0.000000 )
 ( -0.231067  0.000000 -0.427453  0.000000 -0.117800  0.000000 )
     omega( 3) =       1.321104 [THz] =      44.067563 [cm-1]
 ( -0.440024  0.000000  0.236919  0.000000  0.002033  0.000000 )
 ( -0.440074  0.000000  0.236946  0.000000  0.002036  0.000000 )
 ( -0.440074  0.000000  0.237328  0.000000  0.002033  0.000000 )
 ( -0.440784  0.000000  0.236946  0.000000  0.002033  0.000000 )
........
************************************************************************************
While when we perform Fourier transformation with q2r.x, we can get the
interatomic force constants in real space. Then with program matdyn.x, we
can recalculate phonon modes at any q. The results of acoustic branches at
gamma point, are also shown as following (with the acoustic sum rule
applied):

*****************************matdyn.modes***********************
.......................
     omega( 1) =       0.000000 [THz] =      -0.000013 [cm-1]
 ( -0.008792   0.000000     0.001394   0.000000     0.499921   0.000000   )
 ( -0.008792   0.000000     0.001394   0.000000     0.499921   0.000000   )
 ( -0.008792   0.000000     0.001394   0.000000     0.499921   0.000000   )
 ( -0.008792   0.000000     0.001394   0.000000     0.499921   0.000000   )
     omega( 2) =       0.000000 [THz] =       0.000016 [cm-1]
 ( -0.027902   0.000000     0.499217   0.000000    -0.001883   0.000000   )
 ( -0.027902   0.000000     0.499217   0.000000    -0.001883   0.000000   )
 ( -0.027902   0.000000     0.499217   0.000000    -0.001883   0.000000   )
 ( -0.027902   0.000000     0.499217   0.000000    -0.001883   0.000000   )
     omega( 3) =       0.000001 [THz] =       0.000023 [cm-1]
 ( -0.499143   0.000000    -0.027930   0.000000    -0.008701   0.000000   )
 ( -0.499143   0.000000    -0.027930   0.000000    -0.008701   0.000000   )
 ( -0.499143   0.000000    -0.027930   0.000000    -0.008701   0.000000   )
 ( -0.499143   0.000000    -0.027930   0.000000    -0.008701   0.000000   )
***************************************************************************************

My question is that between these two results of eigendisplacement, why they
are so much different and which one is reasonable. Beside, another question
is that, could I square these eigendisplacement, respectively and plus them
together to obtain the vibrational amplitude (that is, SQRT[x^2 + y^2 + z^2]
) .

Thanks in advance!

Sincerely

Kong

-- 
=============================
Yi Kong
Building 23, Room 240
Department of MS&E
Tsinghua University
PR China, 100084

Phone:86-10-6278-1255
Fax:    86-10-6278-1255
Email: kongy03 at mails.tsinghua.edu.cn
or       yi.kong at gmail.com
=============================
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