UNITS LENGTH=A ENERGY=eV

# N atoms
n1: GROUP ATOMS=193
n2: GROUP ATOMS=194
# N2 center of mass 
com_n2: COM ATOMS=n1,n2
# N2 dist
d_n2: DISTANCE ATOMS=n1,n2
# Bottom surface COM
com_surf: COM ATOMS=1-16
# Dist from surf
d_n2_surf: DISTANCE ATOMS=com_surf,com_n2 COMPONENTS NOPBC 

# Coordination
coord: COORDINATION GROUPA=193,194 GROUPB=1-192 R_0=2.5

# Define upper walls 
dwall: UPPER_WALLS ARG=d_n2 AT=2.2 KAPPA=0.2 EXP=2 EPS=0.1          # (distance N-N)

# BIAS
bias: OPES_METAD ...
    ARG=d_n2,coord
    PACE=100 
    BARRIER=1.0
    TEMP=300 
...

# Print
PRINT ARG=* STRIDE=100 FILE=COLVAR 
FLUSH STRIDE=100