# H2 molecule in a big box
#
# Like t24.in, except that GGA is used instead of LDA.
#

ndtset 2  udtset 1 2

# Use the Perdew-Burke-Ernzerhof GGA
 ixc 11

#Definition of the unit cell and ecut, 
#for which one will have to make a convergence study
 acell:? 12 12 12  acell+? 2 2 2 
 ecut 30

#First dataset : find the optimal bond length of H2, and associated total energy
   natom?1 2             # There are two atoms
  ionmov?1 3             # Use the modified Broyden algorithm
   ntime?1 10            # Maximum number of Broyden "timesteps"
  tolmxf?1 5.0d-4        # Stopping criterion for the geometry optimization : when 
                         # the residual forces are less than tolmxf, the Broyden 
                         # algorithm can stop
   xcart?1 -0.7  0.0 0.0 # The starting values of the
            0.7  0.0 0.0 # atomic coordinates
  toldff?1 5.0d-5        # Will stop the SCF cycle when, twice in a row, 
                         # the difference between two consecutive evaluations of
                         # forces differ by less than toldff (in Hartree/Bohr)
   nband?1  1            # Just one band

#Second dataset : get the total energy of the isolated atom
   natom?2 1             # There is one atom
  nsppol?2 2             # Spin-polarized calculation
  occopt?2 2             # Allow occupation numbers to be set by hand
   nband?2 1 1           # Number of bands for spin up and spin down
     occ?2 1.0  0.0      # Occupation numbers for spin up state and spin down state.
  toldfe?2 1.0d-6        # Will stop the SCF cycles when, twice in a row, 
                         # the difference between two consecutive evaluations 
                         # of total energy differ by less than toldfe (in Hartree)
   xcart?2 0.0 0.0 0.0   # The atom is located at the origin
  spinat?2 0.0 0.0 1.0   # Initialisation of spin

#Definition of the atom types
ntypat 1          # There is only one type of atom
znucl 1           # The keyword "znucl" refers to the atomic number of the 
                  # possible type(s) of atom. The pseudopotential(s) 
                  # mentioned in the "files" file must correspond
                  # to the type(s) of atom. Here, the only type is Hydrogen.
                         

#Definition of the atoms
typat 1 1         # For the first dataset, both numbers will be read,
                  # while for the second dataset, only one number will be read

#Definition of the k-point grid
nkpt 1            # Only one k point is needed for isolated system,
                  # taken by default to be 0.0 0.0 0.0

#Definition of the SCF procedure
nstep 10          # Maximal number of SCF cycles
#toldfe is no more defined, as toldff is used above... 
diemac 2.0        # Although this is not mandatory, it is worth to
                  # precondition the SCF cycle. The model dielectric
                  # function used as the standard preconditioner
                  # is described in the "dielng" input variable section.
                  # Here, we follow the prescriptions for molecules
                  # in a big box
