Re: [Phonopy-users] Accuracy of extremely low frequency modes

[Tian L. <ti...@ca...>]

The volume change is not big in my calculation. Usually within the
magnitude of thermal expansion.


On Mon, May 13, 2013 at 10:46 PM, Tian Lan <ti...@ca...> wrote:

> Dear phonopy users,
>
> I am using phonopy to calculate the gruneisen parameters of metal oxides.
> I find in some cases, some acoustic modes have very low frequency around
>  or below 10 to 20 cm-1. In this situation, the modes' response to the
> volume change is kinda strange, i.e, from large volume to small volume, the
> frequency change sometimes does not follow a linear trend, it could go up
> and down and up, for example, 20cm-1->15cm-1-> 21cm-1-> 17cm-1. Therefore,
> there is no way to define a gruneisen parameter. Obviously, in such a low
> frequency, an even small change would lead to a very large gruneisen
> parameter, but does it make any sense?  For higher frequency modes, the
> trend is usually consistent and reasonable.
>
> For those frequencies, I am talking about the commensurate points, for
> example (0.5,0,0) in a 222 supercell. So there is no interpolation issue,
> to my understanding. And since they are perfect crystal structures, in each
> given volume, the atomic structure (with no displace) is perfectly
> relaxed--internal forces=0 for any pair of atoms.
>
> I am wondering whether this is a problem from the structure itself or DFT
> calculation. Or, the small displacement method has the intrinsic numerical
> accuracy limit in such low frequency. Any suggestions would help me
> evaluate my calculation.
>
> Thank you so much.
>
> Best,
> Tian
>
> --
> Lan, Tian
> Ph.D. Candidate, Department of Applied Physics and Materials Science
> California Institute of Technology,
> Caltech M/C 138-78, Pasadena, CA, 91125
>



-- 
Lan, Tian
Ph.D. Candidate, Department of Applied Physics and Materials Science
California Institute of Technology,
Caltech M/C 138-78, Pasadena, CA, 91125