[Pw_forum] About neb and smd

[Stefano Baroni]

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)

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