[QE-users] how to set the electron thermostat fnosee in CP molecular dynamics

[Paolo Giannozzi]

Hi Eduardo

default values should be in a reasonable range, but I am not sure that the
default for "fnosee" was the subject of much thought: 1 THz seems to me a
small value. So don't assume that your estimate is wrong and the default is
right. Note also that there is a small section in the user guide
(CPV/Doc/user_guide.pdf) about Nosé thermostats for electrons.

If you have well separated phonon energies, you may need to use several
Nosé thermostats for ions as well, I think, but I have no experience about
this.

Paolo


On Sun, Oct 14, 2018 at 5:01 PM Eduardo Menendez <eariel99 at gmail.com> wrote:

> Hi fellows,
> I am exploring the Car Parrinello dynamics for a system with hydrogens:
> CH3NH3PbI3. With N-H vibrations faster than 3000 cm^-1 and a
> fluctuating bandgap that may be as small as 1.5 eV , I guess I will need
> to use a thermostat for the electrons, in order not to use a very small
> emass and dt.
> Estimating a minimum electronic frequency sqrt(2Eg/emass), using the
> default emass=400, I get omega_min~0.016 atomic units. For the N-H
> vibrations, the frequency is 3000 cm^ -1~ 0.014 atomic units .
> I read elsewhere than a good frequency for the electron thermostat is 2-3
> times the maximum phonon frequency (90THz), this led me to the value
> fnosee=270.0
> My guess is 270 times larger than the default value fnosep=1.D0. Running
> in a parameter space different to what is tested is generally not a good
> idea.  Am I missing something ?
>
> A related, but independent question is about the ion thermostat.
> The vibrational frequencies of this system are very well separated: ~3000
> cm^-1 for N-H and C-H,
> 900-1200cm^-1 for CH3NH3 molecular vibrations, and <300 cm^-1 for others.
> Hence, should I set an intermediate fnosep=30.0 (1000 cm^-1), or should I
> set a Nose-Hoover chain with frequencies close to every phonon band, e.g.,
> fnosep=90.0 30.0  10.0 3.0
>
> Thanks a lot,
>
> Eduardo Menendez Proupin
> University of Chile
> www.gnm.cl/emenendez
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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