[Wien] top, hollow and bridge interface calculations

[Lyudmila Dobysheva]

Difficult to catch the geometry of the task, but, as far as I understand, the 
symmetry operations in the .struct files are different for the three cases 
(zero xy forces in top and hollow cases and non-zero forces in the bridge 
case). So, I wouldn't compare the total energies of the systems with 
different symmetries, that is with different expansions, different net of 
k-points and other calculation details.

The zero forces in the top case can be a result of the symmetry chosen. The 
equilibrium is probably not stable and a small shift can move the system from 
the equilibrium state, but, of course, the total energy should be higher than 
that of the actual minimum. 
> But we could not reach the zero forces in x and y directions
> for  the case of bridge site.
Why? The program is to move the atoms in the minimum, and energy there should 
be even smaller. I'd expect here that atom in this structure should fall into 
the hollow. 

>       1) Top site 
>        ENE  : **********  TOTAL ENERGY IN Ry =       -92995.521365
>       2) Hollow site
>       ENE   : **********  TOTAL ENERGY IN Ry =       -92995.561715
>       3) Bridge site
>       ENE   : **********  TOTAL ENERGY IN Ry =       -92995.657796

Best wishes
Lyudmila Dobysheva
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