[Pw_forum] questions about some parameter in cp.x

[hyello25 Qgrui]

Hi, everyone,


I took some time to use cp.x in PWSCF. And about some parameters in vc-cp
situation, I am puzzled.

In the IONS namelist,
(1) ion_nstep
     The ion_nstep default value is 1, does it mean calculate 1 time
electronic WFs then then to do ionic step? If so, maybe in an adiabatic
dynamic, 1 step means more time consuming. So, a little larger ion_nstep,
but not to much, may save time to do a MD calculation and keep the dynamic
adiabatic at the same time. Is it right? What possible value we often set
this parameter, 3 or 5 or 10 or larger? How and when we are able to  know
the dynamic process is not adiabatic?

(2) fnosep
    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?
    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?

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?


Thanks in advance.

Bests,

Qian
Dept. of Physics, NJU
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