[Pw_forum] Temperature rescaling in MD calculation

[Changru Ma]

Dear Rui,

On 12 Aug, 2011, at 16:12, DONG Rui wrote:

> Hi, all,
> 
> I am doing MD calculation in QuantumEspresso, and have a question now...
> 
> here are some of the input list
>  &ions
>     ion_temperature = 'reduce-T',
>     tempw=10,
>     delta_t = 1,
>     nraise=10,
>     pot_extrapolation='second-order',
>     wfc_extrapolation='second-order',
>  /
>  
> But I found the rescaling works only when delta_t is a negative number, which is different from what the documentation tells me.

In Doc/INPUT_PW.txt, I find the following lines:

      Variable:       ion_temperature

      Type:           CHARACTER

                                    'reduce-T'    reduce ionic temperature every "nraise" steps
                                    by the (negative) value "delta_t"

and

      Variable:       delta_t

      Type:           REAL

                      if ion_temperature='reduce-T':
                             every 'nraise' steps the instantaneous temperature is
                             reduced by -delta_T (.e. delta_t is added to the temperature)

                      The instantaneous temperature is calculated at the end of
                      every ionic move and BEFORE rescaling. This is the temperature
                      reported in the main output.

                      For delta_t < 0, the actual average rate of heating or cooling
                      should be roughly C*delta_t/(nraise*dt) (C=1 for an
                      ideal gas, C=0.5 for a harmonic solid, theorem of energy
                      equipartition between all quadratic degrees of freedom).

Why delta_t is a negative number is different from the document?

> 
> Anyone has the same problem? And does anyone know how to heat the system up use the method "reduce-T"?

I think "reduce-T" is used to quench the system, it reduces ionic temperature every "nraise" steps by the (negative) value "delta_t".

To heat the system up, I would suggest you to use other thermostats, like "rescaling", "rescale-v", "rescale-T", "berendsen" or "andersen".

Best wishes,
Changru

> 
> Thank you guys!
> 
> Rui _______________________________________________
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---
Changru Ma
SISSA & Theory at Elettra group
email: crma at sissa.it
tel: +39 040 378 7870
http://www.sissa.it/~crma
---

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