How to understand the keywords atoms_from and atoms_to in the colvars??

[ning.zh... at gmail.com]

I am newer for the cp2k. I am studying the metadynamics method. I noted 
that colvars section in the input files includes the keywords atoms_from 
and atoms_to. I don't understand what does it mean? what does the R_0 stand 
for?? For example, see the bold text

&FORCE_EVAL
  METHOD Quickstep
  &DFT
    BASIS_SET_FILE_NAME BASIS_SET
    POTENTIAL_FILE_NAME POTENTIAL
    &MGRID
      CUTOFF 200
    &END MGRID
    &QS
    &END QS
    &SCF
      SCF_GUESS ATOMIC
    &END SCF
    &XC
      &XC_FUNCTIONAL Pade
      &END XC_FUNCTIONAL
    &END XC
  &END DFT
  &SUBSYS
    &CELL
      ABC 5.0 5.0 5.0
    &END CELL
    &COORD
    O   0.000000    0.000000   -0.065587
    H   0.000000   -0.757136    0.520545
    H   0.000000    0.757136    0.520545
    &END COORD
    &KIND H
      BASIS_SET DZVP-GTH-PADE
      POTENTIAL GTH-PADE-q1
    &END KIND
    &KIND O
      BASIS_SET DZVP-GTH-PADE
      POTENTIAL GTH-PADE-q6
    &END KIND
   * &COLVAR*
*       &COORDINATION*
*          ATOMS_FROM 1*
*          ATOMS_TO 2 3*
*          R_0 2.3*
*       &END COORDINATION*
    &END COLVAR
  &END SUBSYS
&END FORCE_EVAL
&GLOBAL
  PROJECT H2O-meta_coord
  RUN_TYPE MD
  PRINT_LEVEL LOW
&END GLOBAL
&MOTION
  &MD
    ENSEMBLE NVT
    STEPS 10000
    TIMESTEP 0.5
    TEMPERATURE 300.0
    &THERMOSTAT
      &NOSE
        LENGTH 3
        YOSHIDA 3
        TIMECON 100.0
        MTS 2
      &END NOSE
    &END
  &END MD
  &FREE_ENERGY
    &METADYN
      DO_HILLS
      NT_HILLS 2
      WW 1.0e-3
      &METAVAR
        SCALE 0.02
        COLVAR 1
      &END METAVAR
      &PRINT
        &COLVAR
           COMMON_ITERATION_LEVELS 3
        &END
        &HILLS
           COMMON_ITERATION_LEVELS 3
        &END
      &END
    &END METADYN
  &END
&END MOTION

Any provides are welcome.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.cp2k.org/archives/cp2k-user/attachments/20150907/712fe760/attachment.htm>