[Pw_forum] Lattice constant for copper

[Krukau, Aliaksandr]

   Dear QE users,
    I apologize if my question seems stupid to you. I am a complete QE newbie
and could not find any answer in the tutorials.
    I am trying to find equilibrium lattice constant for copper with PBE
and ultrasoft pseudopotentials. I ran scf calculations for face-centered cubic
unit cell for a number of lattice constants:
Lattice constant 6.65 a.u. !    total energy              =    
-121.62734203 Ry
Lattice constant 6.66 a.u. !    total energy              =    
-121.62735421 Ry
Lattice constant 6.67 a.u. !    total energy              =    
-121.62734651 Ry
Lattice constant 6.65 a.u. !    total energy              =    
-121.62731956 Ry
    The minimum total energy is for the lattice constant 6.66 a.u.
But this value drastically differs from the published PBE value of 6.86 a.u.
(Phys. Rev. B, vol. 77, 05414 (2008), Table II). It also differs from 
the experimental value of 6.83 a.u. I attach the input file that I used 
for lattice constant 6.66 a.u. below the signature.
    Why is equilibrium lattice constant so low?
Best regards,
Alex Krukau,
Indiana University

------------------------------------------------------
&control
    calculation='scf'
    pseudo_dir = '~/espresso-4.2.1/pseudo/ADDITIONAL',
/
&system
    ibrav = 2,
    celldm(1) = 6.66,
    nat= 1,
    ntyp= 1,
    ecutwfc = 120.0,
    ecutrho = 1000.0
    occupations='smearing', smearing='marzari-vanderbilt', degauss=0.01
/
&electrons
    conv_thr=0.0000001
/
ATOMIC_SPECIES
Cu  63.546 Cu.pbe-n-van_ak.UPF
ATOMIC_POSITIONS
Cu 0.0 0.0 0.0    1   1   1
K_POINTS automatic
12 12 1 0 0 0