[Pw_forum] about forces in CP chemical reactions

[marc at fq.edu.uy]

 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
>> >>
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>> >
>> >
>> >
>> > --
>> > Somesh Kr. Bhattacharya
>> > Post Doctoral Fellow
>> > Room No. 263,
>> > Leonardo Building,
>> > The Abdus Salam International Centre for Theoretical Physics
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> --
> =======================================================================
> 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
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