[QE-users] What's representation 0 in ph.x?

[ykhuang]

Thank you so much.Indeed I used to try to make several single point calculations and fit the data to obtain the frequency. However I am not sure whether the result is reasonable. I'll read the paper you recommend, I believe it will be of help. Best regards. Yike Huang. 
-------- 原始信息 --------发件人： Marzari Nicola <nicola.marzari at epfl.ch> 日期: 2020/2/20  08:24  (GMT+08:00) 收件人： Quantum ESPRESSO users Forum <users at lists.quantum-espresso.org> 主题： Re: [QE-users] What's representation 0 in ph.x? 




You might want to check the 2007 JACS by Dabo et al, where we discuss the independence between adsorption energies and CO vibrational shifts; we also diagonalize the interatomic force constants obtained by finite differences on a local neighborhood.


Nicola

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On 20 Feb 2020, at 01:08, ykhuang <ykhuang at dicp.ac.cn> wrote:





Hello Paulatto,
Thank you for your patient answer. 
Actually in the slab system, the absorption is very weak. Even it is possible that the absorption is still overestimated. So I think it makes sense to treat CO as a decoupled part.
I'll try to calculate the entire matrix or find other ways.
Thank you again. 



Best regards. 


Yike Huang. 



-------- 原始信息 --------
发件人： Lorenzo Paulatto <paulatz at gmail.com> 
日期: 2020/2/20 03:33 (GMT+08:00) 
收件人： users at lists.quantum-espresso.org 
主题： Re: [QE-users] What's representation 0 in ph.x? 



> start_irr = 235
> last_irr = 240

Hello,
I did not understant that the molecule was adsorbed on a surface! In 
this case it makes sense to compute several q-points (not orthogonal to 
the surface!) However, if you set start_irr and last_irr, you are only 
computing a few rows of the dynamical matrix. As the phonons are the 
square roots of the eigenvalues of this matrix, you need the entire 
matrix in order to compute them.

You can compute differents irreducible representations (irr) separately 
and then combine the results together as explained in the Grid_example 
of phonon. But you still need all the matrix. If you have reason to 
think that the CO vibration is completely decoupled from the rest of the 
system, you may try to get its vibrational frequency by doing finite 
differences. It would also be possible to modify phonon to only compute 
the minor of the dynamical matrix that involve CO, then diagonalize it, 
but it is not currently implemented.

hth

-- 
Lorenzo Paulatto - Paris
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