[Pw_forum] Confused on nat definition

[Eyvaz Isaev]

Hi, 

> This may seem silly, but I'm confused as to what
> exactly the number of 
> atoms in a unit cell is (nat).  For example, the
> Silicon example says 
> that nat is only two, yet a diamond structure such
> as this should have 
> much more than two atoms per unit cell.  

Let us consider the diamond case. If you choose as
basis vectors next 3 vectors (which are the standard
choice)

1/2, 1/2, 0
1/2, 0  , 1/2
0  , 1/2, 1/2

you have only 2 atoms in the unit cell
(parallelepiped) spanned by these vectors:
0,     0, 0
1/4, 1/4, 1/4

If you decide to choose as basis vectors next 3 ones

1 0 0
0 1 0
0 0 1

you have 8 atoms in the unit cell which is now a cub.
If your choice is the latter  for CaF2 structure you
will have 12 atoms, but using the former - only 3. 

So, number of atoms (nat) in a unit cell depends on
your unit cell choice defined by 3 basis vectors.

> Is the definition of nat the  number of basis 
> vectors?
To me it is not so clear, but see above.

Bests,
Eyvaz.

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