Energies of Organic Molecules and Atoms in Density Functional Theory

In the assessment of density functional approximations for the exchangecorrelation energy, great weight is usually given to the accuracy of molecular atomization energies, or the enthalpies of formation constructed from calculated atomization energies. Here we show that a recent non-empirical functional, the meta-generalized gradient approximation of Tao, Perdew, Staroverov, and Scuseria, achieves remarkably accurate atomization energies even for the larger organic molecules of the G3-3 test set. But we also present strong evidence that most of the error of previous non-empirical functionals resides in the energy of the free atom, and so cancels out of typical reaction energies. Finally, we suggest that enthalpies of formation calculated without any reference to the free atoms would provide a fairer assessment of the performance of approximate density functionals. keywords: density functional exchange-correlation energy molecular atomization energies enthalpies of formation meta-generalized gradient approximations