[Wien] [Wien2k User Group] Lattice stability in bcc, fcc and hcp and other crystal structures

[Ghosh SUDDHASATTWA]

Hi Stefaan, 
I now noticed the following comment in your reply " Yes (geometry
optimization for bcc or fcc = find the optimal lattice 
constant, no troubles here with internal coordinates)." 

You have mentioned fcc or bcc. Why or? 
I ask this question from the point of view of uranium allotropes. 
It has alpha- beta and gamma -phases. 
So when I report ENE for these allotropes; do I have to do a normal SCF
(with so,sp) or SCF with min_lapw? 
Why should I do a structure optimization because I know they are stable
phases? I might think of only min_lapw. 

In case, I think of uranium in monoclinic lattice which might be a
metastable phase, I need to do a structure optimization. I am still not
clear why should I think of structure optimization for stable phases. 


Suddhasattwa Ghosh 



-----Original Message-----
From: wien-bounces at zeus.theochem.tuwien.ac.at
[mailto:wien-bounces at zeus.theochem.tuwien.ac.at] On Behalf Of Ghosh
SUDDHASATTWA
Sent: Friday, February 19, 2010 3:52 PM
To: 'A Mailing list for WIEN2k users'
Subject: Re: [Wien] [Wien2k User Group] Lattice stability in bcc,fcc and hcp
and other crystal structures

Thank you Stefaan, 
Is there any way out to simultaneously optimize the lattice parameters and
the internal coordinates in w2web? 

SG


-----Original Message-----
From: wien-bounces at zeus.theochem.tuwien.ac.at
[mailto:wien-bounces at zeus.theochem.tuwien.ac.at] On Behalf Of Stefaan
Cottenier
Sent: Friday, February 19, 2010 1:33 PM
To: A Mailing list for WIEN2k users
Subject: Re: [Wien] [Wien2k User Group] Lattice stability in bcc, fcc and
hcp and other crystal structures


> We consider that the stable ground state of a metal is bcc. The fcc 
> structure is the high temperature phase. To calculate the enthalpy of 
> transformation of the fcc to bcc, we consider the following
> 
> Enthalpy of transformation =ENE(fcc)-ENE(bcc)
>
> Now for this above equation, do we have to do a geometry optimization 
> and then calculate the ENE?

Yes (geometry optimization for bcc or fcc = find the optimal lattice 
constant, no troubles here with internal coordinates).

> Now, for example, We want to calculate the ENE of Fe in a monoclinic 
> lattice, then doing a geometry optimization and then finding the ENE has 
> a purpose.
> 
> What about the stable phases? And what about the high temperature phases?

For more complex structures, the unit cell volume and shape (a, b, c, 
alpha, beta, gamma) as well as the internal coordinates have to be 
optimized for all phases.

Mind that in this way you find a zero kelvin tranformation enthalpy, 
which is for a temperature-induced transformation somewhat a flawed concept.

Stefaan

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