[Pw_forum] ionic temperature in CP

[Antonio Tilocca]

>
> Hi All
>
> I am a new user of cp module and I have some questions about running a CPMD
> simulation and using cppp.x.
>
> 1. I tried to run a CPMD job with 33 water molecules in a 1*1*1 nm3 box.
> After minimizing the electrons and ions to their ground states, I started
> 'verlet' dynamics as mentioned in QE user guide, without setting tempw
> ,fnosep and ion_temperature. The tempp value (ionic temperature) directly
> went up to more than 4000 and continued increasing. Did that make sense?
> Shall I turn on temperature controlling at the same time I started verlet
> dynamics? I suppose the system should oscillate around 0K if it has been
> minimized. Or is it because I started the system with a random molecule
> distribution?


To answer your first question: yes, an initial "rocketing" ionic 
temperature is not unexpected if you start from a configuration very far 
from equilibrium, no matter how well you relax the electrons. However, 
also be aware that there can be other causes for this behaviour, such as 
an inappropriate (too long) time step, wrong cell vectors specified, etc. 
In order to rule out the latter, you can try to start from a more stable 
configuration, for instance obtained from a quick relaxation by classical 
MD. In principle you could also try to turn on damped MD to prevent ions 
from moving too fast and too much in the initial stages of the CP dynamics, and 
see a short damped MD run gives you a better starting configuration.


Antonio Tilocca
Chemistry
University College London









>
> 2. I have defined atom species as:
>
>  atomic_number(1) = 1,
>  atomic_number(2) = 8,
>
> in cppp input file.However in all generated files (out/axsf/pdb) all the
> atoms are still recognized as 'H'. What should I do with that?
>
> many thanks
>
> Jin
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