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

[Ghosh SUDDHASATTWA]

Thanks Stefaan, 

1. I want optimized positions in case of experimentally known crystal
structures. Does it really help? Is it worth doing? I may be wrong or may be
doing what is not required. 

2. I want to do only internal coordinates optimization for metastable
phases. Is it incomplete? 

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 Stefaan
Cottenier
Sent: Friday, February 19, 2010 4:51 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


> You have mentioned fcc or bcc. Why or? 

I assumed you mentioned elemental materials (like fcc Ni, or bcc V). 
There are no modifiable internal positions in such a structure, hence 
nothing to do for min_lapw. But :ENE does change if you change the cell 
size and/or shape (here: just the lattice parameter 'a'). Even if you do 
know the experimental value of 'a', it does not necessarily correspond 
to the lowest energy: DFT with presently available functionals is not 
perfect.

> 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? 

It depends on what you want to do. The total energy for the experimental 
structure, for the experimental cell volume/shape with optimized 
positions or for the fully optimized cell are three different 
quantities. It depends on your purpose which one is most suitable.

> Why should I do a structure optimization because I know they are stable
> phases? I might think of only min_lapw. 

See above: the computed answer need not to coincide with the 
experimental answer (sometimes there are moderate deviations, sometimes 
it turns out that either one of both can be plain wrong).

Stefaan

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