[Wien] mixed phase of structure in Wien2k

[Lyudmila Dobysheva]

14.07.2017 23:34, Dr. K. C. Bhamu wrote:
> On Thu, Jul 13, 2017 at 11:45 PM, Dr. K. C. Bhamu <kcbhamu85 at gmail.com 
> <mailto:kcbhamu85 at gmail.com>> wrote:
>     Suppose a material exists in two phases with the ground state energy
>     difference of the order of ~10^-3eV or higher order.
>     Is it possible to simulate a mixed phase calculation for a compound
>     in Wien2k? if not then why?

To construct a good calculational model of the phenomenon, that one is 
interested in, takes 90 % of the time, so no answer to your question, it 
depends on the system.

A calculation is limited by number of atoms in the unit cell, too many 
atoms may be too long a calculation and impossible even at best computers.
If you have two phases of the same composition but different crystal 
structure, let they be monoatomic, you could make a 1x1x100 atoms cell: 
1x1x50 of one atom type, and 1x1x50 of another, but you cannot describe 
here any different structure. So, in order to match different 
structures, you need larger cell, let it be 2x2x100 - 400 atoms it's 
already rather much for WIEN2k.

Then, is the width of layers in the model sufficient for your purpose? 
What happens at the boundary? - it should be relaxed (forces at atoms 
equal to zero), than, all cell parameters should be found (energy 
minimum), forces checked again. Then, look - we made a surface of one 
type, and there can be more energy favourable surface, zigzag, or 
something, this will again require larger cell.
And we have taken only monoatomic phases, if there are more atoms the 
task increase hugely.

Best wishes
   Lyudmila Dobysheva
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