Empirical corrections to density functional theory highlight the importance of nonbonded intramolecular interactions in alkanes.

Energies of alkanes computed with many popular and even newer density functionals are flawed by systematic errors, which become considerable with larger molecules. The same energies, however, are well described by post-Hartree-Fock methods. Similar DFT shortcomings are well documented for cases involving descriptions of intermolecular van der Waals complexes. One solution to the density functional problem is the addition of an empirical correction term, which more accurately models the known R (-6) dependence of van der Waals energies. Here, we present the first empirical correction to DFT parametrized to reproduce experimental energies associated with intramolecular interactions in alkanes. Our training set used only three reactions involving simple linear and branched alkanes and provides a remarkable improvement over conventional DFT methods and empirical corrections optimized for intermolecular interactions. In contrast to many standard density functionals, the intramolecular empirical correction correctly predicts the lowest energy alkane isomer in addition to performing satisfactorily for describing the interaction energies of intermolecular complexes.