[Pw_forum] what's the difference between 'md' in pw.x and cp.x

[vega lew]

Dear sir,

thank you for your so detail answer about the ab initio MD method.

Now, I am much clear about this method.

I have not logged on this maillist because of the blockage of google webpage
(my gmail was blocked too). So sorry for the late appreciation to your kind
reply to my questions.

thank you again

vega

On Tue, Jun 23, 2009 at 8:28 PM, Axel Kohlmeyer <akohlmey at cmm.chem.upenn.edu
> wrote:

> On Tue, 2009-06-23 at 13:34 +0800, vega lew wrote:
> > Dear friends,
> >
> > Thank you for you  time for reading and answering my questions.
> >
> > I realized that the ensemble is much more important than one snap of
> > the whole run, resembling the classical MD to some extent.
>
> there is _no_ difference between classical MD and ab initio MD
> in this respect. the only difference between the two is how you
> get the forces. statistical mechanics doesn't care about the
> forces only about configurations and ensembles.
>
> [...]
>
> > we put a integrated water molecule at certain site on the surface. and
> > then we perform MD calculations of different method, such as CPMD,
> > cp.x, pw.x(calcualtion='md'). I wonder whether the dissociative state
> > could be acheived on the surface at very low temparature (such as 100K
> > or below) via all the MD calculations mentioned above?
>
> you have to look at this from a different perspective. what you will
> see, depends on the potential surfaces of the system, that you are
> studying. running MD allows your system to sample different local
> minima, provided the barriers between them are small enough. if you
> run at low temperature, your MD will not be very different from
> a geometry optimization.
>
> > From your answers, I see cp.x and CPMD of IBM CORP. is using
> > 'Car-Parrinello Molecular Dynamics', while the 'md' calculation in
> > pw.x performs born-oppenheimer MD. Paolo just told us that the
> > born-oppenheimer MD only perform MD at ground state. Does CPMD or cp.x
> > perform the MD calculation at ground state or exited state?
>
> both are methods that look at the ground state wavefunction.
> the CP method uses a "trick" that would allow to propagate
> the wavefunction without having to do a full optimization.
> CP can be faster, but has some approximations and limitations,
> e.g. CP requires a band gap and has not been implemented for
> k-points (at least not in the codes you mentioned, and i don't
> know of a code that has it, even though it should be technically
> possible).
>
> doing excited state calculations with DFT is a different
> problem, as (regular) DFT is a ground state method.
>
>
> cheers,
>     axel.
> >
> > thank you very much for you time
> >
> > best,
> >
> > vega
> >
> >
> > --
> >
> ==================================================================================
> > Vega Lew ( weijia liu)
> > PH.D Candidate in Chemical Engineering
> > State Key Laboratory of Materials-oriented Chemical Engineering
> > College of Chemistry and Chemical Engineering
> > Nanjing University of Technology, 210009, Nanjing, Jiangsu, China
> >
> ******************************************************************************************************************
> > Email: vegalew at gmail.com
> > Office: Room A705, Technical Innovation Building, Xinmofan Road 5#,
> > Nanjing, Jiangsu, China
> >
> ******************************************************************************************************************
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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
> =======================================================================
> If you make something idiot-proof, the universe creates a better idiot.
>
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>



-- 
==================================================================================
Vega Lew ( weijia liu)
PH.D Candidate in Chemical Engineering
State Key Laboratory of Materials-oriented Chemical Engineering
College of Chemistry and Chemical Engineering
Nanjing University of Technology, 210009, Nanjing, Jiangsu, China
******************************************************************************************************************
Email: vegalew at gmail.com
Office: Room A705, Technical Innovation Building, Xinmofan Road 5#, Nanjing,
Jiangsu, China
******************************************************************************************************************
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