Re: [Phonopy-users] Phonon Frequencies off Gamma Point

[Declan S. <dsc...@qu...>]

Hi Aldo,


Thanks for you quick reply. I've compared supercell sizes from the and orthorhombic primitive unit cell to 3x3x1 (54 atoms) and there is a variation in the phonon frequencies at the M point however not at the Gamma point. I am currently running calculations on larger supercells to try and converge the phonon frequencies at the M point.


I have converged the systems very well. Pressure on the unstrained cell is less that 0.01 GPa and the forces on the atoms for strained and unstrained systems are less than 1x10^-6 eV/Angstrom. The energy cutoff is set to 800 eV and the convergence threshold for electronic minimisation is set to 10^-8 with an 18x12x1 k-grid.


The symmetry of the system is broken when under strain though the unstrained system retains it's symmetry after relaxation.


Currently I am only using the GGA functional of Perdew Burke and Ernzerhof. I haven't compared the results to other functionals (LDA or hybrid).


I've also studied the electronic structure and there is no metal to insulator transition. The material in questions is monolayer WS2, one of the transition metal dichalcogenides.


In summary, do you think the phonons at the M point should be well represented and this is probably an issue due to supercell size?


All the best,


Declan



________________________________
From: Aldo Romero <Ald...@ma...>
Sent: 15 January 2017 22:08:55
To: pho...@li...
Subject: Re: [Phonopy-users] Phonon Frequencies off Gamma Point

Declan

It is hard to say... are the phonons well converged (meaning that you have been able to go the largest supercell size and when you compare to a smaller supercell, your phonons looks almost identical?) did you try to compare LDA vs GGA with respect to experiment? If everything is well converged and there is not any weird situation, I guess the problem comes from the functional.. did you try several functionals? Did you converge the crystal structure correctly? Did you check the symmetry after convergence of the geometry?

I do not think, it is a problem of the method, the method is clear, our problem, if the agreement is not good is really from the physics, functional or something even deeper... (things like Metal-to_Insulator transition can really make the problem complicated... is this a strongly correlated material?...

The info you gave does not help us to advise you on the potential problem :)

Cheers

-aldo.


________________________________
Prof. Aldo Humberto Romero
American Physical Society Fellow
Assistant Editor EPJB
Physics and Astronomy Department
West Virginia University
Phone: (304) 2936317
email: alr...@ma...


From: Declan Scullion <dsc...@qu...>
Reply-To: "pho...@li..." <pho...@li...>
Date: Sunday, January 15, 2017 at 12:45 PM
To: "pho...@li..." <pho...@li...>
Subject: [Phonopy-users] Phonon Frequencies off Gamma Point


Dear Professor Togo and Phonopy users,



I'm using phonopy to examine how phonon frequencies change when applying strain to systems. I have studied how the frequencies evolve with strain at the Gamma point and have got some very good agreement with experimental results from collaborators of mine. However, the experimentalists are also examining second order Raman modes such as the 2LA mode at the M point.



My question is, is it also possible to examine how these modes evolve with strain and if so how accurate would the results be? My first set of results do not look very promising and I'm wondering if this is a limitation of the underlying method or more a problem with my analysis.



Many thanks in advance for your help on this.



Kind Regards,



Declan