[Pw_forum] phonon calculation with ultrasoft potentials

[Paolo Giannozzi]

On Feb 11, 2007, at 17:30 , Marcel Mohr wrote:

> I am calculating wurtzite CdSe with the PBE Vanderbilt pseudos
> provided by the pwscf page. Lattice constant is very good. I am
> using a cutoff of 50 Ry, (that is even far beyond convergence)

you may need a very high cutoff on the charge density in
order to get good phonons with ultrasoft pseudopotentials.
Unfortunately this more than offsets the speed advantage
of a lower cutoff on the wavefunctions

>   tr2_ph=1.0d-14

use a smaller threshold: 10^-18 or so

> Maybe the electric field has to be included in the calculation

the LO-TO splitting IS calculated: it is not added to the output
frequencies, but it can be added later using an auxiliary code

Paolo
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Paolo Giannozzi, Democritos and University of Udine, Italy