[Pw_forum] CELLDM and atomic positions

[Guido Roma]

Cristian Rivas wrote:
> 
> Gentlemen:
> 
> In example #1 the CELLDM value is 10.20 and the atomic positions are
> 0.00,0.00,0.00 and 0.25,0.25,0.25 for the silicon simulation.  How is the
> lattice constant of 5.43 Ang. obtained for this bulk silicon
> simulation.  In other words, if I would like to simulate something else
> (say germanium), what kind of values do I have to enter in order to
> specify the correct lattice constant (for example, 5.65 Ang. for
> germanium).


If you want to simulate something else (say Ge) at 0 pressure, and to be
coherent
with the model, you should find the theoretical equilibrium lattice
constant
by minimising E(V) (you can calculate few points to fit with some
standard equation of state). 
Of course your minimum will depend on the energy cutoff, the k-point
sampling, 
the xc-functional/pseudopotential that you use, all ingredients that you
have to keep control on.

Guido 


-- 
Guido Roma <roma at cea.fr> -- CEA-Saclay - DEN/DMN/SRMP Bat.520/130 
Phone: [+33]-1-69085738 -- Fax: ...6867 -- Mobile: [+33]-6-20069085