The ABINIT Users Mailing List ( CLOSED )

[mmikami@rc.m-kagaku.co.jp]

Hello Kwan-woo Lee,

Not a few people may be unfamiliar with the structure of LiBC.
From the preprints (*1,2), it belong to  P6_3/mmc (D_6h^4).
Factor group analysis gives the modes at Gamma-point
(except the acoustic modes):
2A_2u + 2B_1g + B_2u + 2 E_1u +  E_2u + 2 E_2g (see *1)
IR-active mode: 2A_2u + 2E_1u
Raman-active mode : 2 E_2g
You might want to check another direction,
e.g. a direction perpendicular to (0,0,1) to see LO-TO splitting 
fully...
(From the result of the (001) direction, you got A_2u (LO)'s
 and E_2u(TO)'s, but not A_2u (TO)'s and E_2u (LO)'s ...)

From the output of IFC, "Analysis of degeneracies and characters ..."
should be available. So you could mention them next time ...
(I did not understand the comment: "because just 11th and 13th
 modes are IR active in the 001-direction." on which ground ...)

In passing :
It is difficult to assume your calculation conditions (without input !) 
....
(Ecut/kpt is enough ? The crystal structure/internal coordinates
 are well converged for RF calculation ? etc...) So I am not sure
the results are well converged or not. (please check !) In any case,
you might want to read again a thread, which is summarized in
http://www.abinit.org/wws/arc/forum/2002-12/msg00018.html
At the bottom line,  "In practice, people impose the acoustic sum rule
in IFC, which forces the phonon frequencies to be zero at Gamma,
and the other frequencies can be modified accordingly as well."

(* 1) http://arxiv.org/abs/cond-mat/0207683
(* 2) http://arxiv.org/abs/cond-mat/0207299
NB: (* 2) gives 3 A_2u + 2B_1g + B_2u + 3 E_1u + E_2u + 2 E_2g,
(the acoustic modes should be A_2u + E_1u ...)

Regards,
Masayoshi

On 2003.1.28, at 03:09 Asia/Tokyo, Kwan-Woo Lee wrote:

> Dear all,
>
> I got some results about LiBC from ABINT main code and IFC code.
> But two results are different?
>
> At Gamma point,  I got the phonon frequencies by using main code with
> homogenous Electric perturbation. (e.g., for 001 -direction)
>
> {  Phonon at Gamma, with non-analyticity in the
>   direction (cartesian coordinates)  0.00000  0.00000  1.00000
>   Phonon frequencies in cm-1    :
> - -0.211135D+01  0.477301D+01  0.492702D+01  0.169392D+03  0.169400D+03
> -  0.292359D+03  0.292359D+03  0.299125D+03  0.345510D+03  0.345512D+03
> -  0.506804D+03  0.509942D+03  0.827816D+03  0.829173D+03  0.114271D+04
> -  0.114271D+04  0.115339D+04  0.115339D+04 }
>
> I also got the results by using IFC code with DDB coming from the main
> code.  However,
> two results are different.
> {  Phonon at Gamma, with non-analyticity in the
>   direction (cartesian coordinates)  0.00000  0.00000  1.00000
>  Phonon frequencies in cm-1    :
> - -0.486861D-02  0.000000D+00  0.000000D+00  0.169300D+03  0.169302D+03
> -  0.292365D+03  0.292369D+03  0.321218D+03  0.345502D+03  0.345506D+03
> -  0.509444D+03  0.655940D+03  0.823731D+03  0.955713D+03  0.114270D+04
> -  0.114270D+04  0.115338D+04  0.115338D+04 }
>
> According to Factor group analysis,  the results obtained from main 
> code
> look like correct.
> (because just 11th and 13th modes are IR active in the 001-direction.)
>
> Why are two results different from?
> Is the results coming from IFC code correct?
>
> If there is someone having any idea, please let me know it.
>
> thank you,
>
> Kwan-Woo Lee