Density functional study of small molecules and transition-metal carbonyls using revised PBE functionals

We have implemented the generalized gradient approximation (GGA) exchange-correlation functional suggested by Perdew, Burke, and Ernzerhof (PBE) and two of its recent revisions, revPBE and RPBE, in the program ParaGauss, which is a new version of the LCGTO-FF (fitting function) approach designed for parallel computers. With these new functional we investigated a test suite of 19 small molecules as well as the transition-metal carbonyls of Cr, Fe, and Ni using very flexible Gaussian-type orbital (GTO) basis sets. Both revised PBE functionals yield very similar results for the atomization energies of small molecules, improved over those obtained with conventional GGAs (BP, PW91, PBE) as judged by the average absolute error (5 kcal/mol reduced from 8 kcal/mol). Atomization energies of revised PBE functionals are on the average 7 kcal/mol smaller than those of conventional GGAs, consistent with an average bond elongation by about 0.006 Å relative to these GGAs. Revised PBE functionals yield bond distances on average 0.016 Å longer than experiment. With revised PBE functionals, the first carbonyl dissociation energies of Cr(CO)₆, Fe(CO)₅, and Ni(CO)₄ are calculated in excellent agreement with experiment whereas structures are calculated very similar to those of conventional GGAs.