CCL: Fitting IRC data to a polynomial function

Hello  Fellow CCL Members,
 I want to fit an IRC data points attached to a polynomial form to get a
 potential function which I intent to use in calculating WKB barrier penetration
 integral. I have tried several simple polynomials forms up to 5th order
 including some exponentials using Mathematica's NonLinearModelFit but non of
 them seem to work!
 Does any of you guys have an idea what kind of form I should be trying?