[Pw_forum] Some questions about vibrational modes
torstein.fjermestad at kjemi.uio.no
Fri Oct 12 09:50:26 CEST 2012
With the purpose of obtaining the Gibbs free energy, I am computing the
vibrational modes of a system. The system is a microporous zeotype
material with an extra-framework species (Si(OH)4) located in the pore.
The vibrational modes were obtained by first optimizing the structure
with increased accuracy ( forc_conv_thr=1.0d-5, etot_conv_thr=1.0d-6).
Thereafter I did a phonon calculation with the tr2_ph option set to
1.0e-14. I had expected three of the vibrational modes to correspond to
a translation of all atoms in the unit cell in one direction. This
happens indeed for the lowest mode (mode 1, frequency = 3 cm-1) which
corresponds to a translation in the z direction. Modes 3 (frequency = 29
cm-1) and 4 (frequency = 30 cm-1) show a translation of the framework
atoms in the y and x direction respectively, but the Si(OH)4 species is
not translating with the framework atoms. Is such a behavior expected?
When calculating the vibrational contribution to the Gibbs free energy,
one should not include modes corresponding to a translation of the whole
unit cell, but what about cases such as mode 3 and 4 where the Si(OH)4
species is not translating with the rest of the atoms? How are such
cases treated correctly?
Another issue concerns the extra-framework species Si(OH)4. In mode 2
(frequency = 23 cm-1) and 5 (frequency = 51 cm-1) this is being
translated while the rest of the atoms are relatively static. Wouldn't
it be better to treat these modes as translations instead of vibrations?
In that case, how is this done correctly?
Thank you in advance for your help.
University of Oslo,
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