mu0: DISTANCE ATOMS=1,10
vol: VOLUME 

mu1: COMBINE ARG=mu0 COEFFICIENTS=0.5 PERIODIC=NO

EXTERNAL ARG=mu1,vol FILE=potential LABEL=ext

PRINT ...
  STRIDE=1
  ARG=mu1,vol,ext.bias 
  FILE=COLVAR FMT=%6.3f
... PRINT

FLUSH STRIDE=100

ENDPLUMED

