r1:   RMSD REFERENCE=ref.pdb  TYPE=OPTIMAL-FAST NOPBC
r1x:   RMSD REFERENCE=refx.pdb  TYPE=OPTIMAL-FAST NOPBC
r2:   RMSD REFERENCE=ref2.pdb  TYPE=OPTIMAL-FAST NOPBC
#num-r2:   RMSD REFERENCE=ref2.pdb  TYPE=OPTIMAL-FAST NUMERICAL_DERIVATIVES

DUMPDERIVATIVES FILE=deriv ARG=r1,r1x FMT=%12.8f
#DUMPDERIVATIVES FILE=check ARG=r2,num-r2 FMT=%12.8f

PRINT ...
  STRIDE=1
  ARG=*
  FILE=COLVAR FMT=%12.8f
... PRINT

ENDPLUMED