[Pw_forum] Single Cell vs Super-Cell

[Peter O Orondo]

Hi-

 

I am trying to calculate the chemical potential of H in a bunch of metals.
When I use a single cell, I get results that compare to experiment. However,
when I use a 2x2x2 super-cell, I expect to get a more accurate result since
the super-cell should model the physical properties more accurately.
However, the super-cell results don't even have the right shape. The results
look rather random.

 

Any insights into what could be wrong? 

 

Here are more details of what I am doing:

 

To get the chemical potential, I put the a number of H atoms at random into
the available O-sites. Because there can be a large number

H-atoms configurations, I take an average of sample SCF energies.


I build the super-cell using the same techniques as in this forum - and I
check the structure and configuration with Xcrysden before calculating its
energy. Because the lattice constant changes depending on the number of
absorbed H, I relax it at each concentration.

 

My reference configuration is a H2 molecule in a large box (same size as the
supercell)

 

Thanks in advance

 

Peter

Student, EECS, MIT

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