no convergence blues with bulk sio2

[Axel]

hi all!

the following input fails to converge (either with OT set to ON or
OFF).
the exact same setting work very nicely with bulk h2o...
does anybody have an idea where the problem could be?

&GLOBAL
  FFTLIB FFTW3
  PROJECT sio2-liq-qs
  RUN_TYPE MD
  PRINT_LEVEL LOW
&END GLOBAL

&MOTION
  &MD
    ENSEMBLE NVE
    STEPS 1000
    TIMESTEP 0.2
    TEMPERATURE 4000.0
    TEMPERATURE_TOLERANCE 100.0
  &END MD
  &PRINT
    &TRAJECTORY
    &END TRAJECTORY
    &VELOCITIES OFF
    &END VELOCITIES
  &END PRINT
&END MOTION

&FORCE_EVAL
  METHOD QS
  &DFT
    CHARGE 0
    BASIS_SET_FILE_NAME GTH_BASIS_SETS
    POTENTIAL_FILE_NAME GTH_POTENTIALS
    &MGRID
      CUTOFF 300
    &END MGRID
    &QS
      WF_INTERPOLATION ASPC
      EXTRAPOLATION_ORDER 3
    &END QS
    &SCF
      SCF_GUESS ATOMIC
      EPS_SCF      1.0E-6
      MAX_SCF       30
      WRITE_RESTART_EACH 0
      &OT OFF
        MINIMIZER DIIS
        N_DIIS  7
      &END OT
      &OUTER_SCF
        EPS_SCF 1.0E-6
        MAX_SCF 5
      &END
    &END SCF
    &XC
      &XC_FUNCTIONAL PBE
      &END XC_FUNCTIONAL
      &XC_GRID
        XC_SMOOTH_RHO NN50
        XC_DERIV NN50_SMOOTH
      &END
    &END XC
  &END DFT
  &SUBSYS
    &CELL
      ABC 14.257800 14.257800 14.257800
      UNIT ANGSTROM
    &END CELL
    &TOPOLOGY
      COORDINATE PDB
      COORD_FILE_NAME sio2-liq.pdb
    &END
    &KIND Si
      BASIS_SET TZV2P-GTH
      POTENTIAL GTH-PBE-q4
    &END KIND
    &KIND O
      BASIS_SET TZV2P-GTH
      POTENTIAL GTH-PBE-q6
    &END KIND
  &END SUBSYS
&END FORCE_EVAL