      real*8 k1,k2,k3

c global ad hoc parameters
      parameter (k1=16.0)
      parameter (k2=4./3.) 

c reasonable choices are between 3 and 5
c this gives smoth curves with maxima around the integer values
c k3=3 give for CN=0 a slightly smaller value than computed
c for the free atom. This also yields to larger CN for atoms
c in larger molecules but with the same chem. environment
c which is physically not right
c values >5 might lead to bumps in the potential
      parameter (k3=-4.) 

