Constraint

[rupa]

Hi all,
I have some problem with constrained MD of water-dimer with O--O
distance as constraint, which is required to extent up to 5 Å.
However, the required criteria was not satisfied in the simulation,
the extension of O--O distance was beyond 5 Å.
Any suggestion in this regard is appreciated.

Thanks
Rupa

P.S. Input file is provided below for reference.

&FORCE_EVAL
  METHOD Quickstep
  &DFT
    BASIS_SET_FILE_NAME GTH_BASIS_SETS
    POTENTIAL_FILE_NAME POTENTIAL
    &MGRID
      CUTOFF 280
      NGRIDS 1
      REL_CUTOFF 50
    &END MGRID
    &QS
      EPS_DEFAULT 1.0E-12
      EPS_GVG 1.0E-6
      EPS_PGF_ORB 1.0E-6
    &END QS
    &SCF
      EPS_SCF 1.0E-6
      MAX_SCF 350
      SCF_GUESS atomic
    &END SCF
    &XC
      &XC_FUNCTIONAL BLYP
      &END XC_FUNCTIONAL
      &VDW_POTENTIAL
        POTENTIAL_TYPE PAIR_POTENTIAL
        &PAIR_POTENTIAL
         &END PAIR_POTENTIAL
      &END VDW_POTENTIAL
    &END XC
    &POISSON
      PERIODIC NONE
      POISSON_SOLVER MT
    &END POISSON
  &END DFT
  &SUBSYS
    &CELL
      ABC 20.000 20.000 20.000
      PERIODIC NONE
    &END CELL
    &COORD
     H 0.828857203  1.510765153  1.850758135
     O 0.223273347  1.612802255  1.087529046
     H 0.746091252  2.082274953  0.419190658
     O 1.531077052  1.395421022  3.663417772
     H 1.345748937  0.482975975  3.940795058
     H 0.806283419  1.920091495  4.042933787
    &END COORD
    &KIND O
      BASIS_SET TZVP-GTH
      POTENTIAL GTH-BLYP-q6
    &END KIND
    &KIND H
      BASIS_SET TZVP-GTH
      POTENTIAL GTH-BLYP-q1
    &END KIND
    &COLVAR
      &DISTANCE
       ATOMS 2 4
      &END DISTANCE
    &END COLVAR
  &END SUBSYS
&END FORCE_EVAL

&GLOBAL
  PROJECT md
  RUN_TYPE MD
  PRINT_LEVEL LOW
&END GLOBAL

&MOTION
  &MD
    ENSEMBLE NVT
    STEPS  4000
    TIMESTEP 1.0
    TEMPERATURE 300.0
    &THERMOSTAT
      &NOSE
        LENGTH 3
        YOSHIDA 3
        TIMECON 100.
        MTS 2
      &END NOSE
    &END THERMOSTAT
  &END MD
  &CONSTRAINT
    SHAKE_TOLERANCE 1.0E-08
    &COLLECTIVE
      COLVAR 1
      INTERMOLECULAR
      TARGET 5
      TARGET_GROWTH 0.0001
    &END COLLECTIVE
  &END CONSTRAINT
&END MOTION