A DENSITY-FUNCTIONAL STUDY OF CHEMICAL-REACTIONS

Density functional theory (DFT) was used to study reactions involving small molecules. Relative energies of isomers and transition structure s of diazene, formaldehyde, and methylenimine were determined using va rious DFT functionals and results were compared with MP2 and MP4 calcu lations. DFT reaction barriers were found to be consistently lower. Fo r some reactions, such as OH + H-2 --> H2O + H, gradient-corrected fun ctionals predict very low or nonexistent barriers. The hybrid Hartree- Fock-DFT adiabatic connection method (ACM) often provides much better results in such cases. The performance of several density functionals, including ACM, was tested in calculations on over 100 atomization, hy drogenation, bond dissociation, and isodesmic reactions. The ACM funct ional provides consistently better geometries and reaction energetics than does any other functional studied. In cases where both HF and gra dient-corrected DFT methods underestimate bond distances, the ACM geom etries may be inferior to those predicted by gradient-corrected DFT me thods. (C) 1995 John Wiley & Sons, Inc.