ACCURATE DENSITY-FUNCTIONAL THERMOCHEMISTRY FOR LARGER MOLECULES

Density functional methods are combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecu les. Seven different density functionals are assessed for the evaluati on of heats of formation, Delta H-f(0) (298 K), for a test set of 40 m olecules composed of H, C, O and N. The use of bond separation energie s results in a dramatic improvement in the accuracy of all the density functionals. The B3-LYP functional has the smallest mean absolute dev iation from experiment (1.5 kcal mol(-1)).