Re: [Phonopy-users] artificial imaginary frequencies

[Atsushi T. <atz...@gm...>]

Hi,

> I did not quite get what you meant by "commensurate point". Could you
> elaborate a bit more?

A supercell can represent phonon modulations for some q-points. I call
the q-points as commensurate points. For example, 2x1x1 supercell has
the commensurate points of (0, 0, 0), (1/2, 0, 0).

> Also, I actually also tried a 3*3*1 supercell. But it did not improve the
> interpolation.

Can you attach your results, POSCAR, FORCE_CONSTANTS, and configuration file?

Togo

>
> Thank you.
>
> Andy
>
>
> On Sun, Oct 21, 2012 at 11:24 PM, Atsushi Togo <atz...@gm...> wrote:
>>
>> Hi,
>>
>> K-point (I suppose it's hexagonal or trigonal) is perhaps not a
>> commensurate point. So if you really watch if K-point is dyamically
>> stable or not, you should make a supercell of 3x3x1.
>>
>> Togo
>>
>> On Mon, Oct 22, 2012 at 11:29 AM, Caloma Trumica <sp...@gm...> wrote:
>> > Dear all,
>> >
>> > I used a 64-atom supercell to do the phonon calculation using VASP, and
>> > got
>> > the frequencies as shown below:
>> >
>> > 187 f  =    0.871242 THz     5.474172 2PiTHz   29.061488 cm-1
>> > 3.603167
>> > meV
>> > 188 f  =    0.871242 THz     5.474172 2PiTHz   29.061488 cm-1
>> > 3.603167
>> > meV
>> > 189 f  =    0.871242 THz     5.474172 2PiTHz   29.061488 cm-1
>> > 3.603167
>> > meV
>> > 190 f/i=    0.001592 THz     0.010004 2PiTHz    0.053111 cm-1
>> > 0.006585
>> > meV
>> > 191 f/i=    0.001592 THz     0.010004 2PiTHz    0.053111 cm-1
>> > 0.006585
>> > meV
>> > 192 f/i=    0.002956 THz     0.018573 2PiTHz    0.098602 cm-1
>> > 0.012225
>> > meV
>> >
>> > These imaginary frequencies are supposed to be negligibly small.
>> > However,
>> > when I used the phonopy to get the full phonon spectrum in the first
>> > Brillouin zone, I got a large region of imaginary frequencies around the
>> > K
>> > points.
>> >
>> > Any idea on the reason and the solution is appreciated.
>> >
>> > Thank you.
>> >
>> > Andy
>> >
>> >
>> >
>> >
>>
>>
>>
>> --
>> Atsushi Togo
>> http://atztogo.users.sourceforge.net/
>> atz...@gm...
>
>



-- 
Atsushi Togo
http://atztogo.users.sourceforge.net/
atz...@gm...