Thank you Axel, the point is understood.regards.marc
> On Mon, 2009-06-08 at 08:29 -0300, marc at fq.edu.uy wrote: > > hi marc, > >> Hi Somesh >> >> Many thanks for your answer, I have the post that you refer, and read >> the >> paper refered, I understand that they said that a forces difference >> analysis between CP and BO must be done, but also said that they >> supposse >> that for chemical reactions the situtation is more delicate, so my >> question is general about chemical reactions with CP, with criteria are >> used to check that is running fine, just conserved energy, and delta rms >> between forces ? > > this is not the problem. the problem with chemical reactions is that > the electronic structure changes significantly, which in turn will > need the fictitious degrees of freedom (suddenly) to run faster for > a while. because of that, the choice of fictitious mass will affect > the effective barrier that you are seeing. also, you will have some > irreversible transfer of energy from the ionic degrees of freedom > to the fictitious degrees of freedom, which will increase your distance > from the born-oppenheimer surface. the problem is: decreasing the > fictitious mass will keep you closer to the BO surface, but the > CP method will eventually break down and also become very inefficient > due to the requirement of a very small timestep, but to assess how > much a specific choice of fictitious mass (and timestep) will affect > a specific system, is very hard to predict. > > since there are references to the tangney paper. while i believe that > this is a very valuable study, its choice of setups to compare are > very extreme and thus not representative to the typical choices of > parameters which are generally quite safe, unless you have a system > that is not "behaving well". the best choices depend strongly on > the masses of the lightest atoms in the system, so it is impossible > to give a general recommendation other than: try it out, see if it > behaves well (i.e. the fictitious dynamics is stable w/o a thermostat) > and matches OK with some reference BO snapshots). due to more efficient > ways of achieving convergence and effective wavefunction extrapolation > schemes that help to counter one of the biggest disadvantages of BO > dynamics, the choice of BO over CP seems to become more popular these > days. > > cheers, > axel. > >> I hope now is more clear >> >> Best Regards >> >> Marc >> >> Marc Segovia >> Catedra de Quimica Cuantica >> DETEMA- Facultad de Quimica >> Universidad de la Republica >> Uruguay >> ccbg.fq.edu.uy >> >> >> >> > Hii Marc, >> > >> > Am sorry to say that I didn't understand what you want. >> > : >> > >> >> Hi all >> >> >> >> To do a CP on a chemical reaction, I plan follow steps, >> >> minimizing electrons, minimizing ions,randomize positions, finding >> >> maximum time step compatible with ficticious electron mass, then the >> >> recommendations said to verify forces,comparing BO vs CP >> >> forces in several simulations points, as in 2006 Tangley paper, where >> >> delta rms between both forces is calculated. >> >> There is some recommend delta rms value to chemical reactions ? >> > >> > >> > What chemical reaction do you want to study?? >> > >> > >> >> If not, because depends on system, as I understood which must be >> done >> >> is >> >> work with a smaller ficticious mass or do BOMD, could someone clear >> me >> >> this point or signal some seminal job to follow ? >> > >> > >> > http://www.democritos.it/pipermail/pw_forum/2006-April/004004.html >> > >> > >> > >> > >> >> >> >> Thanking in advance your help and support. >> >> >> >> Best Regards >> >> >> >> Marc Segovia >> >> Catedra de Quimica Cuantica >> >> DETEMA- Facultad de Quimica >> >> Universidad de la Republica >> >> Uruguay >> >> ccbg.fq.edu.uy >> >> >> >> _______________________________________________ >> >> Pw_forum mailing list >> >> Pw_forum at pwscf.org >> >> http://www.democritos.it/mailman/listinfo/pw_forum >> >> >> > >> > >> > >> > -- >> > Somesh Kr. Bhattacharya >> > Post Doctoral Fellow >> > Room No. 263, >> > Leonardo Building, >> > The Abdus Salam International Centre for Theoretical Physics >> > Strada Costiera, 11 >> > I-34014 Trieste >> > Italy >> > Phone: +39-040-2240399 >> > _______________________________________________ >> > Pw_forum mailing list >> > Pw_forum at pwscf.org >> > http://www.democritos.it/mailman/listinfo/pw_forum >> > >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum > > -- > ======================================================================= > Axel Kohlmeyer akohlmey at cmm.chem.upenn.edu http://www.cmm.upenn.edu > Center for Molecular Modeling -- University of Pennsylvania > Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323 > tel: 1-215-898-1582, fax: 1-215-573-6233, office-tel: 1-215-898-5425 > ======================================================================= > If you make something idiot-proof, the universe creates a better idiot. > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum >
