[Pw_forum] MD restart algorithm in Quantum Espresso

[Alexey Akimov]

Dear Paolo,

Thank you for your kind reply (I already though that nobody will ever answer this :) ). 
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? In that case will setting the threshold for this problem to very small value help to prevent such divergence (perhaps for the cost of extra iterations)? Also I tried used the potential and wavefunction interpolation during MD simulation (i hope to accelerate calculations) in MD simulations. Do you think this can interfere with the MD restart algorithm?
2) about nstep - do you suggest to look into code?  i just though this might have some faster explanation without digging into code. I think it might be good if someone could make it in accord with the naive expectation that nstep = 1 will make only one MD step, not 2. 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




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-- 
Dr. Alexey V. Akimov

Postdoctoral Research Associate
Department of Chemistry
University of Rochester

aakimov at z.rochester.edu