[Pw_forum] virtual.x and VCA problem

[Yue-Wen Fang]

Dear Prof. Paolo,

Thank you for boosting my thoughts in VCA implemented in Quantum Espresso.
I'll conduct some tests of calculation.

Best regards!
Yue-Wen FANG

2013/11/19 Paolo Giannozzi <paolo.giannozzi at uniud.it>

> 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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> >         http://pwscf.org/mailman/listinfo/pw_forum
> >
> >
> >
> >
> >
> > --
> >
> ------------------------------------------------------------------------------------------------------------
> > 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<http://clpm.ecnu.edu.cn/>
East China Normal University  <http://english.ecnu.edu.cn/>
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