Assessment of Basis Set and Functional Dependencies in Density Functional Theory: Studies of Atomization and Reaction Energies

In search of the optimal combination of basis set and exchange-correlation potential, we have investigated the dependence of the atomization energies (D0) and reaction enthalpies (ΔH) for a set of 44 molecules using gradient-corrected density functional theory. Of the six functionals tested, those that include a portion of the exact (Hartree−Fock) exchange perform best and yield D0 values that generally lie within 3−5 kcal/mol of the experimental value. For the functionals in which pure DFT exchange is employed, the errors in D0 are instead on the order of 8−10 kcal/mol. Conversely, reaction enthalpies show slightly better agreement with experimental results when pure DFT exchange is employed. For both D0 and ΔH the four Gaussian basis sets 6-31G(d,p), 6-311G(d,p), cc-pVDZ, and cc-pVTZ show similar behavior. The most accurate predictions were obtained using the largest cc-pVTZ basis. There is a significant variation in these energies obtained with the various functionals depending on the basis set employed.