! E04: the O2 molecule in its triplet ground state.
! A molecule in a periodic code = one molecule in a big empty box.

&CONTROL
  calculation       = 'scf'
  prefix            = 'o2'
  outdir            = './tmp/'
  pseudo_dir        = './pseudo/'
  verbosity         = 'high'
  tprnfor           = .true.      ! check the residual force at d = 1.21 A
/
&SYSTEM
  ibrav             = 1           ! simple cubic box
  celldm(1)         = 20.0        ! box edge in bohr (~10.6 A); vacuum size is a convergence parameter
  nat               = 2
  ntyp              = 1
  ecutwfc           = 60          ! oxygen is a hard element; higher cutoff than Si
  ecutrho           = 480         ! 8x rule again
  assume_isolated   = 'mt'        ! Martyna-Tuckerman correction for the periodic images
  nspin             = 2           ! spin-polarized
  tot_magnetization = 2.0         ! CONSTRAIN the cell moment to 2 uB: the triplet.
                                  ! (two separate Fermi levels are used, up and down)
  occupations       = 'smearing'  ! the degenerate pi* level needs fractional occupations
  smearing          = 'gaussian'
  degauss           = 0.001       ! tiny width: only for numerical stability
/
&ELECTRONS
  conv_thr          = 1.0d-8
  mixing_beta       = 0.3         ! gentler mixing; molecular levels shift easily
/

ATOMIC_SPECIES
  O  15.9994  O.pbe-n-kjpaw_psl.1.0.0.UPF

! absolute Cartesian coordinates; 1.21 A is the experimental bond length
ATOMIC_POSITIONS (angstrom)
  O  0.000  0.000  0.000
  O  0.000  0.000  1.210

! a molecule has no band dispersion: one k-point at Gamma.
! the 'gamma' keyword also switches on the real-wavefunction speedup.
K_POINTS gamma