[Pw_forum] lattice constant from unit cell vs. from supercell

[Chad Junkermeier]

Hi,
   I am trying to perform a set of SCF calculations on graphene with F  
atoms attached.  If I do this using the unit cell, everything seems to  
work correctly, and the equilibrium lattice constant for the system  
seems to be correct.  If, in stead, I use a 2x2 supercell I find that  
the equilibrium lattice constant is not the same (or even close) to  
the unit cell result.
   The only differences between the two sets of calculations are the  
number of atoms and the value used for alat, and the number of kpoints.

The supercell case has the following output (no analog of which is  
found in the unit cell case):

     Found symmetry operation: I + (  0.0000 -0.5000  0.0000)
      This is a supercell, fractional translation are disabled


Can anyone please tell me where I am going wrong in the supercell case?

Thank you.


Chad Junkermeier, Ph.D.
NRC Post-Doctoral Fellow
Naval Research Laboratory
4555 Overlook Ave, S.W.
Washington DC 20375
Office: (202) 404-4635
Fax: (202) 767-1165
Email:  junkermeier(at)bloch.nrl.navy.mil


Note that the value for celldm(3) in both of these files was computed:  
celldm(3) = 46.5/alat

The commands for the unit cell case (alat = 4.80):

&CONTROL
calculation = "relax",
pseudo_dir  = "/app/espresso/pseudo",
outdir      = "/workspace/junky/lattice_top/tmp",
/
&SYSTEM
ibrav       = 4,
celldm(1)   = 4.800,
celldm(3)   = 9.68750000000000000000,
nat         = 3,
ntyp        = 2,
ecutwfc     = 300.0,
occupations = "smearing",
smearing    = "methfessel-paxton",
degauss     = 0.050,
/
&ELECTRONS
conv_thr    = 0.000001,
mixing_beta = 0.30,
/
&IONS
bfgs_ndim         = 3,
pot_extrapolation = "second_order",
wfc_extrapolation = "second_order",
/
ATOMIC_SPECIES
C  6.0 C.pbe-van_ak.UPF
F  9.0 F.pbe-n-van.UPF
ATOMIC_POSITIONS crystal
C   0.0000000    0.0000000    0.0000
C   1.00/3.00  2.00/3.00    0.0000
F   0.0000000    0.0000000    0.050000
K_POINTS automatic
8 8 1  0 0  0





The commands for the supercell case (alat = 4.65, celldm(1)=2*alat):

&CONTROL
calculation = "relax",
pseudo_dir  = "/app/espresso/pseudo",
outdir      = "/workspace/junky/twotwoF01/tmp",
/
&SYSTEM
ibrav       = 4,
celldm(1)   = 9.300,
celldm(3)   = 10.00000000000000000000,
nat         = 10,
ntyp        = 2,
ecutwfc     = 300.0,
occupations = "smearing",
smearing    = "methfessel-paxton",
degauss     = 0.05,
/
&ELECTRONS
conv_thr    = 0.000001,
mixing_beta = 0.3,
/
&IONS
bfgs_ndim         = 3,
pot_extrapolation = "second_order",
wfc_extrapolation = "second_order",
/
ATOMIC_SPECIES
C  6.0 C.pbe-van_ak.UPF
F  9.0 F.pbe-n-van.UPF
ATOMIC_POSITIONS crystal
C       0.0     0.0     0.0
C       0.1666666666666667      0.3333333333333333      0.0
C       0.0     0.5     0.0
C       0.1666666666666667      0.8333333333333333      0.0
C       0.5     0.0     0.0
C       0.6666666666666667      0.3333333333333333      0.0
C       0.5     0.5     0.0
C       0.6666666666666667      0.8333333333333333      0.0
F       0.0     0.0     0.05
F       0.1666666666666667      0.3333333333333333      0.05
K_POINTS automatic
4 4 1  0 0  0










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