[Pw_forum] static dielectric constant (esp_0)

[Stefano Baroni]

Sorry everybody. In a previous post I said:

On Jul 23, 2008, at 11:48 AM, Stefano Baroni wrote:

> example 31 deals with bulk silicon, a bulk, nonpolar, system for  
> which \eps_\infty=\eps_0
> the result you get is what you should expect. technically, the  
> vanishing forces on atoms are due to the vanishing ionic (Born)  
> effective charges, which in turn is due to the tetrahedral point  
> symmetry of atomic sites

which is obviously wrong. (in binary semicondoctors the point symmetry  
of ionic sites is also tetrahedral, but the effective charges are  
obviously nonzero)

in the diamond structure, vanishing effective charges results from  
dynamical charge neutrality [Z(1)+Z(2)=0], which says that the total  
force exerted by a uniform electric field on the crystal is zero, plus  
crystal symmetry, which requires Z(1)=Z(2). (here, of course, indeces  
(1) and (2) refer to the two lattice sites in the unit cell).

again, sorry about that
SB

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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