zero step size in cell optimisation

[isaac sugden]

Hello All,

I have recently studied a graphitic material with CP2K, described by this 
input files first section:
 &GLOBAL
   PRINT_LEVEL  MEDIUM
   PROJECT_NAME MAY
   RUN_TYPE CELL_OPT
 &END GLOBAL
 &MOTION
   &GEO_OPT
     OPTIMIZER  CG
   &END GEO_OPT
   &CELL_OPT
     OPTIMIZER  CG
     TYPE DIRECT_CELL_OPT
   &END CELL_OPT
 &END MOTION
 &FORCE_EVAL
   METHOD  QS
   STRESS_TENSOR  ANALYTICAL
   &DFT
#     WFN_RESTART_FILE_NAME  MAY-RESTART.wfn
     &SCF
       MAX_SCF             1000
       SCF_GUESS  ATOMIC
       &OT
        LINESEARCH 3PNT
       &END OT
     &END SCF
     &QS
       EPS_DEFAULT     9.9999999999999998E-13
       EPS_GVG_RSPACE     9.9999999999999995E-07
       EPS_PGF_ORB     9.9999999999999995E-07
     &END QS
     &XC
       DENSITY_CUTOFF     1.0000000000000000E-10
       GRADIENT_CUTOFF     1.0000000000000000E-10
       DENSITY_SMOOTH_CUTOFF_RANGE     1.0000000000000000E-10
       TAU_CUTOFF     1.0000000000000000E-10
       &XC_FUNCTIONAL  NO_SHORTCUT
         &PBE  T
         &END PBE
       &END XC_FUNCTIONAL
         &vdW_POTENTIAL
         DISPERSION_FUNCTIONAL PAIR_POTENTIAL
         &PAIR_POTENTIAL
            TYPE DFTD2
            R_CUTOFF 15.0
            SCALING 1.0
        &END PAIR_POTENTIAL
      &END vdW_POTENTIAL
the cell optimisations see convergence of step size to 0, whilst retaining 
high forces:
  Max. step size             =         0.0000000000
  Conv. limit for step size  =         0.0030000000
  Convergence in step size   =                  YES
  RMS step size              =         0.0000000000
  Conv. limit for RMS step   =         0.0015000000
  Convergence in RMS step    =                  YES
  Max. gradient              =         0.0102471915
  Conv. limit for gradients  =         0.0004500000
  Conv. for gradients        =                   NO
  RMS gradient               =         0.0009970071
  Conv. limit for RMS grad.  =         0.0003000000
  Conv. for gradients        =                   NO
Vibrational analysis sees some negative frequencies. 
Has anyone else experienced something similar? ~I have looked at using BFGS 
as the optimizer but this sees an increase in energy. 

many thanks,
Isaac
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