The variation of correlation energies with bond distances of various first row diatomic molecules has been studied. Self-consistent field and complete active space self-consistent field potential curves of these molecules have been calculated. Exact potential energy curves are constructed from experimental data using the Rydberg-Klein-Rees method. With appropriate definitions, the dynamical and nondynamical correlation energies are obtained and the variation of these with bond distance is calculated. Two definitions of nondynamical correlation are examined. Classifying the angular correlation as dynamical seems to be a better way to partition the correlation energy. The correlation functionals of density functional theory, VWN, LYP, and P86, are also evaluated and compared with the ab initio dynamical correlation energies. LYP appears to give the closest agreement with the dynamical correlation energy.