14.07.2017 23:34, Dr. K. C. Bhamu wrote:
On Thu, Jul 13, 2017 at 11:45 PM, Dr. K. C. Bhamu <kcbham...@gmail.com <mailto:kcbham...@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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