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. -------------- next part -------------- A non-text attachment was scrubbed... Name: md.tar.gz Type: application/x-gzip Size: 10088 bytes Desc: not available Url : http://www.democritos.it/pipermail/pw_forum/attachments/20120820/00d8f862/attachment-0001.bin -------------- next part -------------- > 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
