Force Constants

 I'm trying to compute the force constant for the superoxide ion (O2-)
 using MP2/6-31G*. using Gaussian 03. The value I get after running a
 frequency calculation is 11.03 mDyne/A. This converts to 68.9 eV/A^2.
 The experimental value (using spectroscopic constants) is about 37.8
 eV/A^2. Clealy, I am quite off the mark. I suspect I am not
 interpreting the gaussian results correctly.  Could anyone throw some
 light on whether I'm doign something wrong,  and if so, what ? Any
 help is will be appreciated.