[Pw_forum] strange results for phonon frequencies.
Elie M
elie.moujaes at hotmail.co.uk
Sun Sep 2 06:09:50 CEST 2012
Dear all,
Sorry to ask so many questions about computing phonon frequencies but recently i have got some strange results for one of the systems I am working on. My work involves computing phonon frequencies for a 42 carbon at the Gamma point so in total 126 irreducible representations. in a previous calculations, my input was:
SCF FILE
/
&system
ibrav= 0, celldm(1) =1.88972599, nat=42,
ntyp= 1, ecutwfc =32, ecutrho=128, occupations='smearing', smearing='mp',degauss=0.01
/
&electrons
conv_thr=1.D-6,
mixing_beta=0.7D0,
diago_david_ndim=2,
ATOMIC_SPECIES
C
12.0107 C.pbe-rrkjus.UPF
...
PH FILE:
Phonon
calculation for GB60 at gamma point
&inputph
tr2_ph=1.0d-15,
prefix='GrBo42ph',
fildvscf='GB60dvscf',
elph=.true.,.....
The results obtained were:
omega(
1) = -4.450324 [THz] = -148.446823 [cm-1]
omega( 2) = -3.144068 [THz] = -104.874835 [cm-1]
omega( 3) = 2.963314 [THz] = 98.845504 [cm-1]
omega( 4) = 3.454797 [THz] = 115.239637 [cm-1]
omega( 5) = 3.908880 [THz] = 130.386191 [cm-1]... omega(**) = 47.380063 [THz] = 1580.428796 [cm-1]
omega(**) = 47.573318 [THz] = 1586.875087 [cm-1]
omega(**) = 47.821300 [THz] = 1595.146853 [cm-1]
omega(**) = 48.119632 [THz] = 1605.098137 [cm-1]
Where the 2 negative frequencies,as professor Gionazzi repeated several times, could be due to the violation of the ASR rule.
For my own piece of mind I repeated the calculation with a better accuracy where ecutrho=256 (8 times ecutwfc which should be the case if an ultrasoft potential is used)and tr2_ph =10^(-17) although mixing_beta was set equal to 0.3 this time. I expected better results but what I got was the following:
omega(
1 - 1) = -77614121.2
[cm-1] --> A' I+R
omega( 2 - 2) =
-13157144.1 [cm-1] --> A' I+R
omega( 3 - 3) =
-5577066.3 [cm-1] --> A' I+R
omega( 4 - 4) =
-4332492.8 [cm-1] --> A' I+R
omega( 5 - 5) =
-4094962.7 [cm-1] --> A' I+R
omega( 6 - 6) =
-3368158.6 [cm-1] --> A' I+R
omega( 7 - 7) =
-2781643.0 [cm-1] --> A' I+R
omega( 8 - 8) =
-2403295.2 [cm-1] --> A' I+R
omega( 9 - 9) =
-1777809.3 [cm-1] --> A' I+R
omega( 10 - 10) =
82.5 [cm-1] --> A'' I+R
omega( 11 - 11) =
85.8 [cm-1] --> A'' I+R... omega(120 -120) =
2301243.8 [cm-1] --> A' I+R
omega(121 -121) =
2494732.0 [cm-1] --> A' I+R
omega(122 -122) =
2788571.0 [cm-1] --> A' I+R
omega(123 -123) =
3135326.3 [cm-1] --> A' I+R
omega(124 -124) =
3630869.3 [cm-1] --> A' I+R
omega(125 -125) =
8808450.4 [cm-1] --> A' I+R
What is the reason for those LARGE negative values at the beginning and the LARGE ones at the end? Is it because mixing_beta was changed?
Thanks for the help
Elie KoukaesUniversity of NottinghamNG7 2RDUK
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