[Pw_forum] About neb and smd

[Stefano Baroni]

Nichols: your question makes sense indeed. NPT dynamics has indeed  
been designed by Parrinello and Rahman (and somewhat refined by  
Wentzcovitch) to facilitate the transition from one crystal structure  
to another. However, although the relative enthalpies of the two  
phase can obviously be estimated from the simulation, the *barrier*  
between them have little (if anything) physical, for the reasons that  
I tried to explain the other time. As a matter of fact, in order to  
observe a transition in NPT MD run it is known that one has to  
overpressurize the sample ... Hope this helps. Stefano



On Feb 14, 2006, at 3:47 AM, Nichols A. Romero wrote:

> Stefano,
>
> This may seem like an ignorant question, but please bear with me I am
> just a lowly post-doc :^)
>
> Your previous statement on simulating first-order phase transitions
> make sense. However, I am confused at to where NPT type calculations
> fit into all of this. Are they not often used for simulating
> pressure-induced phase transitions? Consider for example, the
> following article:
>
> hcp-to-bcc pressure-induced transition in Mg simulated by ab initio
> molecular dynamics
> R. M. Wentzcovitch
> Phys. Rev. B 50, 10358-10361 (1994)
>
> Or does your statement pertain to simulating the real ("direct")
> dynamics that occurs during the first-order phase transition?
>
> I hope my question makes sense.
>
> Bests,
> On 1/30/06, Stefano Baroni <baroni at sissa.it> wrote:
>> Dear Huiqun Zhou: sorry for the (very) late reply
>>
>>
>> On Dec 31, 2005, at 4:59 AM, Huiqun Zhou wrote:
>>
>> Dear list-users,
>>
>> Could anyone give me a pointer whether it's theoretically  
>> reasonable to
>> investigate solid state
>> transition induced by pressure using NEB or string method?
>>
>> In principle, it is. In practice, I would have strong reservations.
>>
>> The energy difference between two different phases is an extensive  
>> quantity,
>> hence the jump from one structure right to another would require  
>> to overcome
>> an infinite energy barrier in the thermodynamic limit. The way  
>> structural
>> transitions occur in nature is through nucleation: an energy  
>> fluctuation of
>> the order of (K_B T) determines the change of structure in a small  
>> portion
>> of the crystal which then gradually increases its size until it  
>> "eats" the
>> whole system. In order to accomodate two different structures to  
>> coexist, a
>> lot of defects are produced at the interface and the disturbance  
>> may extend
>> far from the nucleaton center due to the long-range nature of the  
>> resulting
>> elastic interactions. The energy barrier results from a delicate  
>> balance
>> between the energy gain due to the phase transformation and the  
>> energy loss
>> due to the creation of defects. As, noticed, the size of the  
>> perturbed
>> region may be large, so that a direct simulation would require large
>> supercells.
>>
>>
>>  Any references are appreciated.
>>
>> I am not directly aware of any, but I would not be surprised if a few
>> existed. Nor would I be surprised if most of them were wrong, just  
>> because
>> of the temptation of simulating a direct jump between two phases just
>> ignoring the nucleation process.
>>
>>
>>  Happy New Year!
>>
>> All the same to you
>> Stefano
>>
>> ---
>> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
>> Trieste
>> [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
>>
>> Please, if possible, don't  send me MS Word or PowerPoint attachments
>> Why? See:
>> http://www.gnu.org/philosophy/no-word-attachments.html
>>
>>
>>
>>
>
>
> --
> Nichols A. Romero, Ph.D.
> 1613 Denise Dr. Apt. D
> Forest Hill, MD 21050
> 443-567-8328 (C)
> 410-306-0709 (O)
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

Please, if possible, don't  send me MS Word or PowerPoint attachments
Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html



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