[Pw_forum] one more question about NEB and CP

[lfhuang]

Dear vega lew:
    Harmonic Transition State Theory (hTST) can help you to calculate the attempt frequency which can be obtained if the vibrational frequencies of the reactant and saddle-point states are known. 
Reference: H.Eyring, J.Chem.Phys. 3,107 (1935) (Or you can call it "quantum mechanically modified TST"). Its classical limit is the Vineyard form: G.H.Vineyard, J.Phys.Chem.Solids 3,121 (1957)
However, the calculation of phonons maybe time consuming, so empirical values for attempt frequency should be prefered if you just want to make an estimation. The usually used attempt frequency range from 1.0*10^12 to 5.0*10^12 (or the optical mode frequency as prof. N. Marzari said). This should yield quite reasonable results because the jump frequency is just linearly proportional to the attempt frequency, and the jump frequency is exponentially dependent on the activation energy and temperature. 
And from the Arrhenius formula, an reaction event should not happen within several pico-seconds under 1000 K with E_ac=0.6 eV, and T>800 K should be required for the reaction with 0.2 eV. In order to estimate, you need just plot the Arrhenius formula in Gnuplot or Origin: v=v_attemp*exp(-11594E_ac/T) with E_ac in eV unit.
Best Wishes!
Yours Sincerely
L.F.Huang

> From: vega lew 
> Subject: Re: [Pw_forum] one more question about NEB and CP
> To: PWSCF Forum 
> Message-ID:
> 	
> Content-Type: text/plain; charset="iso-8859-1"
> 
> Dear friends,
> 
> thank you for all the help.
> 
> I see arrhenius formula is suitable for my question. But I am wondering how
> to determine the attempt rate for specific system?
> 
> Do you think 300K is suitable for the reaction with activation energy of 0.2
> eV?
> 
> And in my case, the product state is lower than reactant state for about 0.4
> eV, i.e. if the reaction happens, there is 0.6 eV barrier for
> the product state to go back to the reactant state. Do you think the
> dynamical equilibria of the reaction could see in some ps simulation
> of pw.x?
> 
> best wishes,
> 
> vega
> 
> On Mon, Nov 23, 2009 at 6:23 AM, Nicola Marzari  wrote:
> 
> > vega lew wrote:
> > > Dear all,
> > >
> > > I used to perform NEB calculations to estimate the energy barrier for
> > > certain chemical reaction.
> > > The calculated activation barrier is about 0.2 eV. I want the see the
> > > reaction process during some pico-second of CP calculations.
> > > Do you think the reaction could happen in such a short time?  and what
> > > temperature is suitable for jumping over the saddle point?
> > >
> > > thank you for reading
> > >
> > > best wishes,
> > >
> > > vega
> >
> >
> > Hi Vega,
> >
> > try to work this out by yourself, looking at the arrhenius formula ; put an
> > attempt rate comparable to say an optical phonon. But the answer is
> > probably yes,
> > especially if you are a bit over room temperature (500-600K).
> >
> >                nicola
> >
> >
> > --
> > ---------------------------------------------------------------------
> > Prof Nicola Marzari   Department of Materials Science and Engineering
> > 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
> > tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
> >
> > _______________________________________________
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> > http://www.democritos.it/mailman/listinfo/pw_forum
> >
> 
> 
> 
> -- 
> ==================================================================================
> Vega Lew ( weijia liu)
> Graduate student
> 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
> 

------
======================================================================
L.F.Huang(黄良锋) ph.D candidate, DFT and phonon physics
======================================================================
Add: Research Laboratory for Computational Materials Sciences,
Instutue of Solid State Physics,the Chinese Academy of Sciences,
P.O.Box 1129, Hefei 230031, P.R.China
Tel: 86-551-5591464-328(office)
Fax: 86-551-5591434
Web: http://theory.issp.ac.cn (website of our theory group)
http://www.issp.ac.cn    (website of our institute)
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