[Pw_forum] Negative freuqencies for extrapolating points

[Francesco Pelizza]

In my experience, increasing also the convergence for forces and total 
energy of the scf cycle you perform before the neb.x run, helps a lot to 
clean negative frequencies.


Then, all other cutoffs and settings that increase amount of memory and 
cpu time, can helps, sometimes significantly, sometimes their not enough.


Before the phonon, a proper lattice relaxation with higher parameters 
than default value for forces, total energy and eventually even electron 
convergence (as also wavefunction, charge density, etc..), helps much 
more to bring your model to the highest converged condition than just 
increasing parameters thresholds on phonon that can make the phonon a 
jurassic waiting.


These are the reflections  helped me more


Francesco

Strathclyde




On 25/08/16 15:03, Elio Physics wrote:
>
> Dear all,
>
>
> I have calculated the frequencies for a structure. All frequencies 
> were positive except for a few "extrapolated" points very close to the 
> Gamma point which were negative in the range of -1.45 cm^-1. Due to 
> this the dispersion curve near the Gamma point looked a bit funny (but 
> looks pretty good elsewhere).  I tried the calculations with 2 
> different phonon grids. Any reason why this is happening? Do i have to 
> lower  tr2_ph more (it is already 10^(-19)!)
>
>
> Any help would be appreciated.
>
>
> Elio
>
> Fed Uni of Rondonia
>
> Brazil
>
>
>
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