[Pw_forum] about forces in CP chemical reactions

[Axel Kohlmeyer]

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
> >>
> >> _______________________________________________
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> >>
> >
> >
> >
> > --
> > Somesh Kr. Bhattacharya
> > Post Doctoral Fellow
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> 
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-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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