[Pw_forum] questions about some parameter in cp.x

[Paolo Giannozzi]

hyello25 Qgrui wrote:

> (1) ion_nstep  

"ion_nstepe". Used only for dynamics on the Born-Oppenheimer surface,
i.e. with electrons on the ground state. It is not used for the
Car-Parrinello dynamics. I am not even sure it is really used

>     As mentioned in the Doc/input_CP, fnosep means the oscillation freq. 
> of the nose thermostat. While in the Doc/INPUT.HOWTO, I was told 
> QNP=2*g*Kb/Omega^2, where Omega is the typical ionic frequency. Dose 
> fnosep and Omega represent the same thing?

you are answering your own question: no.

>     If a system was calculated with the phonon frequency at Gamma point, 
> such as 100cm^-1, 200cm^-1, 500cm^-1 respective, How should I set the 
> fnosep? is fnosep=7.5 ok, because of 3 THz=100cm^-1 and Omega should be 
> half of the maximum phonon freq?

the best way to know is to try

>  In the SYSTEM namelist,
> (3)ecfixed, qcutz, q2sigma
>     What is the general value we often use in vc-MD process?  I think 
> this question is stupid, these parameters value may come from 
> experience. While how can I know, the value I set is ok or bad?

some explanation here:

http://www.quantum-espresso.org/wiki/index.php/Methodological_Background#Stress

P.
-- 
Paolo Giannozzi, Democritos and University of Udine, Italy