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

[elbuesta at icqmail.com]

Hi Dr. Junkermeier,
 
 If you check with xcrysden, you will see that the first input does not reproduce a graphene layer because the carbon atoms are overlapped. I believe the first input should be something like:
 
 &CONTROL
 calculation = "relax",
 pseudo_dir  = "/app/espresso/pseudo",
 outdir      = "/workspace/junky/lattice_top/tmp",
 /
 &SYSTEM
 ibrav       = 4,
 celldm(1)   = 4.80,
 celldm(3)   = 9.6875,
 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    0.000000
 C   0.3333333    0.6666666    0.000000
 F   0.0000000    0.0000000    0.050000
 K_POINTS automatic
 8 8 1  0 0  0
 
   
 
  But even using this input, the position of the F atoms is not the same compared to the second input. I don't know how many F atoms you want on top of the carbon, so I suggest you to use Xcrysden and than decide how do you exactly want your structure.
 I hope it helps. All best.
 
 Fabio Negreiros Ribeiro
 Departamento de Física da Universidade Federal de Minas Gerais - Brasil
 
 
 -----Original Message-----
 From: Chad Junkermeier <junkermeier at bloch.nrl.navy.mil>
 To: pw_forum at pwscf.org
 Sent: Wed, Mar 17, 2010 4:45 pm
 Subject: [Pw_forum] lattice constant from unit cell vs. from supercell
 
              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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