[Pw_forum] vc-relax and dt

[Gabriele Sclauzero]

Il giorno 15/lug/2010, alle ore 15.38, Masoud Nahali ha scritto:

> Dear PWscf Users
> 
> Is using and setting the variable "dt" in the vc-relax calculation necessary or not ?

As far as I can understand, it is necessary only if you use damped dynamics. If you use BFGS algorithm (which is the default), it shouldn't.

> In the forum are many different suggestions. Also, how can one determine which value of
> the variable "max_seconds" is sufficient and safe.

Have you read the documentation??? This variable tells the code after which (elapsed) time the calculation should be stopped smoothly. It is needed only if you have some time constraints on how long you can run your job uninterruptedly (for instance, due to walltime restrictions in a supercomputing center). If you set the variable to a value below the walltime the code should stop and save everything before your job gets kicked out. Then you can restart your calculation safely. It does not affect the convergence of your calculation in any way.

> when one does not use the "max_seconds" the vc-relax calculation may takes huge time without changing the cell 
> after a specific time. 

This sound quite strange, the two things should not be related. I could barely believe they are...

> A comprehensive example for vc-relax calculation should be very useful.


Please have a look at examples/{example03,VCSexample} and here:
http://www.quantum-espresso.org/wiki/index.php/QESB09
http://www.fisica.uniud.it/~giannozz/QE-Tutorial/

> 
> Many Thanks
> Masoud
>    
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§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne

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