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["zhangtingPKU" <zhangting@pku.edu.cn>]

Xavier Gonze,您好！

        Thanks for your reply. I'll think the whole problem over and will ask 
further questions. 

    Regards.

                                                     Zhang ting  

======= 2006-08-04 15:27:00 您在来信中写道：=======

>Dear ZhangtingPKU,
>
>On 04 Aug 2006, at 10:01, zhangtingPKU wrote:
>
>> Dear all
>>
>>          I'm interested in the phonon-phonon interaction problem, like  
>> the Umklapp process. To handle such a process like an optic phonon  
>> spliting into two acoustic phonons with ab-initio method, we need  
>> the 3rd order expansion of the potential:
>>
>>          U3= (1/6)sigma(B*R1*R2*R3)
>>
>>         And in particular, the 3rd order anharmonic coefficients  
>> are the most important.
>
>Right.
>
>> I've read the paper "Density-functional approach to nonlinear- 
>> response coefficients of solids" (PRB 39,13120, Year 1989), and I'm  
>> interested in whether the ABINIT code can perform such a third- 
>> order coefficients calculation.
>
>Unfortunately, such a calculation using the 2n+1 theorem, is not  
>implemented in the present version of ABINIT. Several other 3rd order  
>coefficients can be computed
>based on the 2n+1 theorem in ABINIT, but not the anharmonic phonon  
>coefficients.
>
>> This kind of task is not included in the tutorials, and I have made  
>> some attempt but it failed. Can someone tell me how can I perform  
>> this calculation? Thanks!
>>         In attachment are my test files and the error message in  
>> log file. I don't clearly know what's the meaning of the keywords  
>> like "rf2phon" and "rf3phon". Does this mean the third phonon or  
>> the 3rd order perturbation?
>
>These input variables are not used in the present version of ABINIT.
>The point is that, in the precursor of the DFPT part of ABINIT,  
>called "RESPFN" (dating back 1990-1995), there was an implementation
>of the computation of phonon-phonon interaction coefficients, for the  
>Gamma point only (see the results
>published in Phys. Rev. A 52, 1096 (1995), esp. page 1111 ). But,  
>when the ground state ABINIT (v1) was merged
>with RESPFN, some parts of RESPFN were left "temporarily" ... and  
>never included again ...
>So, some input variables are present, but not the computation.
>
>If you are interested in the decay of a particular high-symmetry  
>phonon, it should be possible to extract the
>needed coefficients from coupled linear-response and finite- 
>difference calculations :
>compare the interatomic force constants for a geometry containing
>this high-symmetry phonon in a frozen state to those derived from an  
>unperturbed geometry calculation ...
>
>Best wishes,
>Xavier Gonze

= = = = = = = = = = = = = = = = = = = =
                        

　　　　　　　　致
礼！
 
                                 
　　　　　　　　zhangtingPKU
　　　　　　　　zhangting@pku.edu.cn
　　　　　　　　　　2006-08-05