[Pw_forum] (no subject)

[Stefano de Gironcoli]

Raghani Pushpa wrote:

>Dear users,
>I am calculating the phonon  spectrum of a bct structure. When I calculate 
>the force constant file with q2r.x program, it comes out to be the same 
>with zasr=.true. and zasr=.false. It should actually be different in the 
>two cases. Where do you think the problem is arising? 
>
>I calculated the force constants for gaas which is given as the examples 
>in pwscf and with the same program q2r.x, the force constants are 
>different with zasr=.true. and zasr=.false. i.e. there is no problem with 
>the program as such.
>
>Pushpa
>
Accurate caculation of Zstars and dielectric constants requires a more 
accurate BZ sampling than other properties.
zstar=.true. option is provided because experience showed that good 
phonon frequencies can be obtained from Zstars computed with non 
converged BZ sampling (that therefore do not satisfy the acustic sume 
rule accurately) by imposing this rule by hand.

Maybe in your case the Acustic Sum Rule for the effective charges is 
already well satisfied by the  Zstar computed directly by ph.x  (as it 
should be when one makes calculation with large enough number of 
k-points) and the zasr=.true. option does not change significantly the 
computed values of Zstar.

stefano