d1: DISTANCE ATOMS=1,2
d2: DISTANCE ATOMS=3,4

METAD ...
  ARG=d1,d2 SIGMA=0.1,0.1 PACE=2 HEIGHT=0.5
  FMT=%6.2f
  GRID_MIN=0,0 GRID_MAX=2,2 GRID_BIN=4,4
  GRID_WFILE=grid GRID_WSTRIDE=2
  LABEL=meta1
...

PRINT ARG=d1,d2,meta1.bias FILE=COLVAR
