[Phonopy-users] Fwd: How from a single unit cell (not supercell) phonon dispersion appears?

[Santu B. <san...@gm...>]

---------- Forwarded message ----------
From: Santu Baidya <san...@gm...>
Date: 23 August 2013 12:44
Subject: Re: [Phonopy-users] How from a single unit cell (not supercell)
phonon dispersion appears?
To: Alejandro Rébola <ar...@ui...>


Dear phonopy users,

I have got an explanation for the single unit cell phonon calculation using
phonopy and I need to know if that is correct or not.

>From phonopy manual I found that
 A force and a second-order force constant [image: \Phi_{\alpha \beta}] are
given by

[image: F_\alpha(jl) = -\frac{\partial V }{\partial r_\alpha(jl)}]
where l is an unit cell and

[image: \Phi_{\alpha\beta}(jl, j'l') = \frac{\partial^2 V}{\partial
r_\alpha(jl) \partial r_\beta(j'l')} = -\frac{\partial
F_\beta(j'l')}{\partial r_\alpha(jl)},]
For single unit cell it will be l = l'

VASP gives Force on atoms although at single q-point (by DFPT method) also
mentioned in phonopy manual.

phonopy calculates Force constant matrix from the Forces.

Then Dynamical Matrix form will be

D (jj', q) = 1/√(mj mj' )  [image: \Phi_{\alpha \beta}] (j0, j'0)    exp(
iq . [r(j'0) - r(j0)])



And then from here we get dynamical matrix for pair interactions and then
solve the dynamical eqn of motion.

We get then phonon frequencies at other q-points also.


Could any one please tell me if this is correct explanation or not.


Thanks.

Santu Baidya










On 23 August 2013 11:19, Santu Baidya <san...@gm...> wrote:

> Dear Alejandro,
>
> Thanks for your reply . Ofcourse it helps me to understand.
>
>  Can I ask you then if I use original unit cell (not supercell) whose
> lattice parameters ~ 8 Angstrom (large enough) and do not make any
> supercell  and do a VASP calculation (GAMMA point ) to get forces within
> that  range.
>
> And then I use phonopy I shall get frequencies even at other K-ponts.
>
> Is that right?
>
> Could you please explain it little more.
>
> Thanks again.
>
> Santu Baidya
>
>
>


-- 
*"The happiest people do not always have the best of all,* * they simply
appreciate  what they find on their way!!!" SANTU
*



-- 
*"The happiest people do not always have the best of all,* * they simply
appreciate  what they find on their way!!!" SANTU
*