Oxidative addition of the ethane CC bond to Pd. An ab initio benchmark and DFT validation study

We have computed a state‐of‐the‐art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane CC bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta‐GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian‐type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four‐component all‐electron approach. Our best estimate of kinetic and thermodynamic parameters is −10.8 (−11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and −4.5 (−6.8) kcal/mol for the reaction energy (zero‐point vibrational energy‐corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal‐ d ‐electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta‐GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from −0.2 to −3.2 kcal/mol. Interestingly, the well‐known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by −0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only −0.1 kcal/mol. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1006–1020, 2005