[Pw_forum] About neb and smd

[Nichols A. Romero]

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
>
>
>
>


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