[Pw_forum] Single Cell vs Super-Cell

[Peter O Orondo]

Thanks Prof. for that insight. 

 

 

Peter O Orondo wrote:

 

> 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?

 

hard to say. The fact that you get results that compare well to

experiments doesn't imply that those results are correct. In

simple metals, if you put one H atom per unit cell you simulate

a hydride rather than H dissolved into a metal. If you increase

the size of the supercell, you should notice that your results

(e.g. the formation energy) converges to some value, which

should be good for a isolated impurity. Convergence may be

quite slow, though, depending upon what you calculate and

in which system

 

Paolo

-- 

Paolo Giannozzi, Democritos and University of Udine, Italy

 

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