[Pw_forum] virtual.x and VCA problem

[Paolo Giannozzi]

On Tue, 2013-11-19 at 23:16 +0800, Yue-Wen Fang wrote:

> I still don't how to set it while using VCA. For example, in SrTiO3
> bulk material, if I want to mix O with N(namely nitrogen dopping) ,
> after getting a mixed PP's with virtual.x, the atomic weight of oxygen
> in ATOMIC_SPECIES tag is hard to determine if I intend to do phonon
> calculations.

VCA may (or may not) work for the electronic structure, but I
doubt it will work for phonons, especially if the difference 
in mass is significant and especially for phonon branches that 
have significantly different energies in the case of pure atoms. 
You may assume that the mass of the virtual atom is equal to the
weighted average of the two masses, with no warranty. 
Code "dynmat.x" allows to use the force constants (that do not 
depend upon masses) of the virtual crystal into a supercell of
"real" atoms with different masses. Not sure it is suitable for
your case, and not sure still works: it was written 20 or so 
years ago, used for phonons in GaAS/AlAs superlattices and 
GaAsAl alloys in which such "mass approximaton" works well:
Phys. Rev. B 41, 3870 (1990), Phys. Rev. Lett. 65, 84 (1990).

P.
> 2013/11/19 Paolo Giannozzi <paolo.giannozzi at uniud.it>
>         On Tue, 2013-11-19 at 08:52 +0800, Yue-Wen Fang wrote:
>         
>         > In quantum espresso, what is determined by atomic weight?
>         
>         
>         molecular dynamics and dynamical matrices (phonons)
>         
>         > In other words, why the atomic weight can be set freely by
>         users.
>         
>         such possibility may be useful if you want to take into
>         account
>         isotopic effects, for instance
>         
>         P.
>         
>         > Thank you.
>         >
>         >
>         > 2013/11/18 Paolo Giannozzi <paolo.giannozzi at uniud.it>
>         >         On Mon, 2013-11-18 at 19:03 +0800, Yue-Wen Fang
>         wrote:
>         >
>         >         > Additionally, when using mixed PPs generated by
>         virtual.x,
>         >         > how to determine the atomic weight?
>         >
>         >
>         >         atomic weights are used only for molecular dynamics
>         >         and phonon calculations. Unless you want to perform
>         >         such calculations using VCA (likely not a good idea)
>         >         you may choose whatever atomic weight you like
>         >
>         >         P.
>         >         --
>         >         Paolo Giannozzi, Dept.
>         Chemistry&Physics&Environment,
>         >         Univ. Udine, via delle Scienze 208, 33100 Udine,
>         Italy
>         >         Phone +39-0432-558216, fax +39-0432-558222
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>         > --
>         >
>         ------------------------------------------------------------------------------------------------------------
>         > Yue-Wen FANG, PhD candidate
>         > Key Laboratory of Polar Materials and Devices, Ministry of
>         Education
>         > East China Normal University
>         >
>         >
>         >
>         >
>         >
>         >
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>         --
>          Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
>          Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
>          Phone +39-0432-558216, fax +39-0432-558222
>         
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> 
> 
> 
> -- 
> ------------------------------------------------------------------------------------------------------------
> Yue-Wen FANG, PhD candidate
> Key Laboratory of Polar Materials and Devices, Ministry of Education
> East China Normal University 
> 
> 
> 
> 
> 
> 
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-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222