[CPMD-list] About thermostat

[cpmd-list at cpmd.org]

Dear Dr Gao,

> I'm now doing NVT and find that the temperature varies
> from 150K to 450K when the thermostat is 300K. I don't
> know how to decrease the variation around 300K. Please
> help me. Below is my input:
> 
> &CPMD
> RESTART ALL LATEST
> MOLECULAR DYNAMICS
> QUENCH ELECTRONS BO
> TRAJECTORY SAMPLE
> 10
> NOSE IONS
> 300 2000
> NOSE ELECTRONS
> 0.0001 6000
> NOSE PARAMETERS
> 8 8 4 40 125 1
> EMASS
> 800
> &END
> 
> Yi Gao
> 
> Institute of Theoretical and Computational Chemistry,
> Nanjing Univ. P.R.China
> http://itcc.nju.edu.cn

First of all, how many atoms do you have in your system? Like replied
earlier on the list
(http://www.cpmd.org/pipermail/cpmd-list/2002-November/000499.html),
the intrinsic fluctuations (in NVE ensemble) depends on the number of
degrees of freedom. Secondly, are you sure that there are ionic
vibrations at 2000 cm^-1 to which the Nose thermostat can couple to?
Hopefully you are aware of the possible implications caused by using
thermostats, please see e.g. the books by Allen & Tildesley and
Frenkel & Smit. Well, in metallic systems this might be the only
possibility to conserve the total energy, I admit.

  Thirdly, many people on the list have sent inputs with a very small
value for the target of the fictitious kinetic energy. Please notice
that if it's too small the energy drift is INCREASED [see the paper by
P. Blochl, M. Parrinello, PRB 45 (1992) 9413], so please be careful
with these parameters.

    Greetings,

       apsi

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