[Pw_forum] MD restart algorithm in Quantum Espresso

[Paolo Giannozzi]

On Aug 19, 2012, at 24:09 , Alexey Akimov wrote:

> 1) about first part - yes, i meant BOMD calculations. So are you  
> saying that the difference arises because of the convergence  
> criteria for the  electronic problem at restart and during  
> continuous MD run are different?

actually the convergence threshold on self consistency is the same,  
but the starting
threshold for diagonalization differs in a restarted run from that of  
a continuous run.
I have verified (see attached) that apart from this small difference,  
the restart works
properly. You can perform one step at the time by setting nstep=0.  
While I agree
that this is a funny behavior, I think that there are more important  
things to be fixed
in BOMD right now

P.

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> I suspect that the code performs 1 extra step at the end of  
> simulation. In case of long trajectory e.g. nstep=100 this will not  
> make a difference, but if i want to do the sequence : 1 MD step -  
> (do something) - restart and do one more step - etc..., then 1  
> extra MD step at each calculation will double the computational  
> expenses. So is it possible to change such behavior e.g. in next  
> versions?
>
> Thank you,
> Alexey
>
> ----- Original Message -----
> From: "Paolo Giannozzi" <giannozz at democritos.it>
> To: "PWSCF Forum" <pw_forum at pwscf.org>
> Sent: Friday, August 17, 2012 1:25:40 PM
> Subject: Re: [Pw_forum] MD restart algorithm in Quantum Espresso
>
> About the first point: are you referring to MD on the ground state
> ("Born-Oppenheimer" MD), done with PWscf (not Car-Parrinello)?
> I think that the restart does the correct thing, i.e. use the Verlet
> algorithm as if the run was not interrupted, but there might be
> some subtle differences in how the threshold for convergence is
> calculated, that are more than sufficient to have the two MD
> simulations diverge.
>
> About the difference between nstep and the number of steps actually
> performed: it shouldn't be difficult to follow variable "nstep" and
> see why
> this happen
>
> P.
> On Aug 1, 2012, at 24:51 , Alexey Akimov wrote:
>
>> I tried to perform md simulations in Quantum Espresso in 2
>> different ways:
>> 1) simply run a single continuous trajectory (e.g. 10 steps)
>> 2) run first step of MD as a new calculation (restart_mode =
>> from_scratch, default)
>>   and run all other (remaining 9) steps as restarts (restart_mode =
>> restart), doing this for every step
>>
>> As a result after a few steps the total energies, atomic forces and
>> position started to deviate between two approaches.
>>
>> I suspect that some information from the previous step may be
>> dropped during the restart (e.g. by using the first-order Euler
>> scheme instead of the Verlet algorithm), eventually leading to such
>> divergence. So my question is: what is restart algorithm for MD in
>> quantum espresso and is there any possibility to use method 2 (see
>> above) without accumulation of the integration errors?
>>
>> Also can someone clarify why if i specify nstep = 1 in my input
>> file (request to perform a single MD step), the program actually
>> makes 2 MD steps?
>>
>>
>> Thank you,
>> Alexey
>>
>>
>>
>>
>> -- 
>> Dr. Alexey V. Akimov
>>
>> Postdoctoral Research Associate
>> Department of Chemistry
>> University of Rochester
>>
>> aakimov at z.rochester.edu
>> _______________________________________________
>> Pw_forum mailing list
>> Pw_forum at pwscf.org
>> http://www.democritos.it/mailman/listinfo/pw_forum
>
> ---
> Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
>
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>
> -- 
> Dr. Alexey V. Akimov
>
> Postdoctoral Research Associate
> Department of Chemistry
> University of Rochester
>
> aakimov at z.rochester.edu
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum

---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222