Using correlation consistent basis sets from double through quintuple zeta quality, potential energy functions have been calculated for the electronic ground states of the first row homonuclear diatomic molecules B2, C2, N2, O2 and F2 using single and double excitation configuration interaction (HF + 1 + 2, GVB + 1 + 2, and CAS + 1 + 2) wave functions. Spectroscopic constants have been calculated for each species and compared to experiment. The dependence of the calculated spectroscopic constants on systematic extensions of the one-particle basis set are, in general, found to be very regular. By fitting the directly calculated values with a simple exponential function, accurate estimates of the complete basis set (CBS) limit for E(e), D(e), and r(e) have been obtained for each level of theory. The estimated CBS limits are compared to the available experimental results, and the intrinsic errors associated with each theoretical method are discussed. In addition, the accuracy of the internally contracted CAS + 1 + 2 method is compared to conventional uncontracted calculations using large basis sets. For B2, a full CI calculation have been carried out for D(e) with the correlation consistent double zeta basis set and is compared to the CAS + 1 + 2 method using both a supermolecule and separated atom approach for the dissociated limit.