[Pw_forum] Single Cell vs Super-Cell

[Paolo Giannozzi]

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