[Pw_forum] relaxation and TS

[vega lew]

  Dear sir,

Thank you for your detailed explanation. My understanding on the TS is 
improved.

best

vega

On 09/30/10 16:21, Stefano Baroni wrote:
>
> On Sep 29, 2010, at 5:32 PM, vega lew wrote:
>
>> Through the relaxation of a oxygen molecule on a surface by QE4.2, it 
>> dissociated and adsorbed on the surface. The scf energy of each step 
>> has decreased in compared with the previous step until the relaxation 
>> completed. Now, can one say that there is not any TS for O2 
>> dissociation here ?
>>
>>> Probably not. The potential energy surface might be not very 
>>> corrugated and the energy barrier for the dissociation might be not 
>>> very large, so that the dissociation of O2 could happen in a relax 
>>> calculation. I think there might be a very minute TS in the 
>>> dissociation process.
>
> here, I beg to differ. If a state, say B, can be reached from another 
> state, say A, following a path along which the energy always 
> decreases, this means *by definition* that there is *no* energy 
> barrier between A and B. I admit that if the energy barrier is so 
> "minute" as to be smaller that the numerical accuracy of the 
> minimization algorithm, it can escape proper detection. (notice that 
> this can be said of most, if not all, numerical computations of any 
> quantity)
>
>>
>> Or is the reaction spontaneous ?
>
> It is spontaneous
>
>>> If the total energy of system goes down continuously and the 
>>> adsorption energy of dissociated state is lower than the one of 
>>> molecular state, you could say the reaction is favorable. Whether it 
>>> is spontaneous, it is depend on how large of the energy barrier.
>
> in the reported conditions, the energy barrier is zero
>
>>> If there was a TS in this procedure (dissociation and adsorption of 
>>> O2) would the optimization algorithm of QE4.2 stop the work or 
>>> continue ?
>
> In the presence of an energy barrier, the system would stay stuck in 
> the starting configuration
>
> Stefano
>
>> If there is only one bond broken in the process you considered, you 
>> could try constrained optimization technique. For you information, 
>> please refer to B.Hammer (Hammer, B.; Jacobsen, K. W.; Norskov, J. K. 
>> Phys. Rev. Lett. 1992, 69, 1971) or PJ Hu's work. (Alavi, A.; Hu, P.; 
>> Deutsch, T.; Silvestrelli, P. L.; Hutter, J. Phys. Rev. Lett. 1998, 
>> 80, 3650). Or recent developed method by ZP Liu (J. Chem. Theory 
>> Comput. 2010, 6, 1136–1144 )
>
> If I have not misunderstood some important point, no constrained 
> minimization or other advanced techniques are needed here.
>
> Stefano B
>
> ---
> Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center - Trieste
> http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / 
> stefanobaroni (skype)
>
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-- 

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

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