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

[Ghosh SUDDHASATTWA]

Hi Stefaan, 
Thanks for the reply. I am knowing more and more about the questions I have
asked myself. 
To your following querie
" And which 'a' and 'c' lattice parameters are you going to take for  
hcp-U?" 

I assume U is in the hcp Zr ( we know the lattice parameters of Zr in hcp
from ICSD) 
The atomic positions are 0.3333333 0.6666667 and 0.25 
So I imagine U to have the same lattice parameters and then I do min_lapw. 
I optimize only the internal coordinates; I try to minimize the forces
assuming my lattice parameters are same. 
Can I make such an assumption? Please pardon me if I am doing any basic
mistake in assuming this. 

To your first comment on the perfect agreement between theory and
experiment, I come back to the comment you made in the mail before last. 
"It all depends on your problem -- I don't think this mailing list can
answer that for you.
Don't put the horse behind the carriage: first define for yourself what is
the question you want to answer. Then examine what is the 'best' 
computational strategy to answer that question (where 'best' is your
compromise between speed, accuracy, effort and unavoidable limitations). 
Finally, carry out the calculations, and don't make interpretations that are
not allowed under the approximations that you have adopted." 

I beg your pardon Stefaan I am fairly a new user of Wien2k and I would like
to know what would be the best computational strategy. 
1. min_lapw only 
2. volume optimization only 
3. both 

If I go for 1 alone, I assume that my lattice parameters are optimized 
If I go for 2 alone, I need not do min_lapw 
If I go for 3, it would take a large amount of computational time and
effort. 

My limited knowledge in Wien2k makes me go for 1 only that is going for
min_lapw. 
Since these phases are complicated, especially beta-U, which have 8
nonequivalent positions, I cannot go for 3. 
Is my strategy ok? 


Any way, thanks Stefaan for all your replies and suggestions. I know more
about min_lapw than before after discussing with you on this topic. 

Suddhasattwa 


-----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: Sunday, February 21, 2010 1:43 AM
To: wien at zeus.theochem.tuwien.ac.at
Subject: Re: [Wien] [Wien2k User Group] Lattice stability in bcc,fcc and hcp
and other crystal structures


> The stable phases of uranium are C-centered orthorhombic (alpha),
primitive
> tetragonal(beta) and fcc (gamma).
> Getting the atomic positions of these phases from ICSD, I can calculate
the
> ENE (ground state energy). I need not do any min_lapw since these are
stable
> phases.

That last statement is a peculiar one. It implies you assume that any  
structure taken from ICSD will give you zero forces...? And no further  
energy lowering if you (slightly) change cell size and shape...? A  
perfect agreement between theory and experiment? Hmmm...

> Now I imagine uranium in hcp phase of any metal like any refractory
element
> (which are usually in hcp) (Blaha's papers on hcp metals are a good
starting
> point). I replace the hcp crystal structure with uranium only. Then I
> calculate the ENE.
> This is the ENE of uranium in the metastable phase. I guess I am right.

And which 'a' and 'c' lattice parameters are you going to take for  
hcp-U? You can make a smart guess, but in the end you'll find yourself  
making a 2D scan of energies as a function of 'a' and 'c' to find the  
optimal prediction for the cell size and shape.

Stefaan

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