# [Pw_forum] how to determine the mass of virtual-atom generated by virtual.x

Stefano de Gironcoli degironc at sissa.it
Thu Oct 25 17:19:23 CEST 2007

A virtual atom is a ficticious object that you introduce with some
purpouse...

Which mass (if any) is more appropriate depends on what is your purpouse.
If you want to relax the system any mass is good since it is not used.

If you plan to calculate phonons of a material then you should use the
mass appropriate for it. In certain cases (GaAlAs alloys or
superlattices for instance) it is a very good approximation to calculate
the interatomic force constants with ana average atom and then study the
big systems introducing the disorder by assigning the "true" Ga and Al
masses to electronically equivalent virtual atoms (this approximation is
called Mass Approximation)
[see for instance
S.~Baroni, S.~de~Gironcoli, and P.~Giannozzi,
{\sl Phonon dispersions in Ga$_{x}$Al$_{1-x}$As alloys},
Phys.\ Rev.\ Lett.\ {\bf 65}, 84 (1990) ;
E.~Molinari, S.~Baroni, P.~Giannozzi, and S.~de~Gironcoli,
{\sl Effects of disorder on the Raman spectra of GaAs/AlAs superlattices},
Phys.\ Rev.\ B {\bf 45}, 4280 (1992);
G.~Scamarcio, L.~Tapfer, W.~K\"onig, A.~Fisher, K.~Ploog, E.~Molinari,
S.~Baroni, P.~Giannozzi, and S.~de~Gironcoli,
{\sl Infrared reflectivity by TO phonons in (GaAs)$_{m}$(AlAs)$_{n}$
ultrathin superlattices},
Phys.\ Rev.\ B (RC) {\bf 43}, 14754 (1991);
C.~Bungaro, and S.~de~Gironcoli,
{\sl Ab initio study of phonons in Al$_x$Ga$_{1-x}$N alloys},
Appl.\ Phys.\ Lett.\ {\bf 76}, 2101 (2000). ]

I'm sure that there are systems where no choice is reasonable.

stefano

wangjunjie1981_0 wrote:
> Dear Stefano