[Pw_forum] re. Atomic_Positions

[Yong Jiang]

Dear All,

I am new to PWscf. The manual seems too simple to clerify everything. For 
example, I am very confused with Atomic_Positions, expecially the difference 
between {alat} and {crystal}.

To be specific, are the following two settings equivalent to generate the SAME 
fcc unit cell, EXCEPT that the total energy by setting 2 would be four times of

that by setting 1, i.e. Etot(2)=4*Etot(1)?
 
1.
&system
  ibrav=2, nat=  1
ATOMIC_POSITIONS
  Cu 0.00 0.00 0.00

2.
&system
  ibrav=1, nat=  4,
ATOMIC_POSITIONS {alat}
  Cu 0.00 0.00 0.00
  Cu 0.5 0.5 0.00 
  Cu 0.00 0.5 0.5
  Cu 0.5 0.00 0.5 

If we want express the same unit cell in ATOMIC_POSITIONS {crystal}, what would

it be? 

Thanks in advance for help

Yong