After exploring the underlying physics of the cluster-surface analogy, we introduce the concept of "metallic atom". Case studies of the Ni-CO cluster as a model of CO/Ni chemisorption are carried out with UHF,/STO-3G of the so-called atomic ζa and metallic ζm, where ζa is a basis function optimized from the ground state of a Ni atom, while ζm is a modifier of ζa based on the free-electron theory in solid state physics. The calculation results of ζa and ζm are elucidated in the light of more rigorous cluster calculations in the literature.

We derive the form for an exact exchange energy density for a density decaying with Gaussian-like behavior at long range. Based on this, we develop the X3LYP (extended hybrid functional combined with Lee-Yang-Parr correlation functional) extended functional for density functional theory to significantly improve the accuracy for hydrogen-bonded and van der Waals complexes while also improving the accuracy in heats of formation, ionization potentials, electron affinities, and total atomic energies [over the most popular and accurate method, B3LYP (Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional)]. X3LYP also leads to a good description of dipole moments, polarizabilities, and accurate excitation energies from s to d orbitals for transition metal atoms and ions. We suggest that X3LYP will be useful for predicting ligand binding in proteins and DNA.

For a variety of density functional theories, we examined the ground-state properties of the water monomer (geometry, vibrational frequencies, dipole moment, polarizability) and dimer (geometry, vibrational frequencies, bond energy, and barrier heights for the transition states for the interchange of hydrogen atoms within the dimer). Thus, we considered LDA (SVWN), seven pure GGA methods (BLYP, BP86, BPW91, PWPW, mPWPW, PBEPBE, and XLYP), and eight hybrid GGA methods (BH&HLYP, B3LYP, B3P86, B3PW91, PW1PW, mPW1PW, PBE1PBE and X3LYP). We find that the best overall performance is given by X3LYP, a hybrid method using a modified GGA constructed from a linear combination of the Becke and Perdew GGAs. Comparing with the exact values, the errors in X3LYP for the water dimer are 0.05 kcal/mol (bond energy), 0.004

Periana et al. [Science 1998, 280, 560] previously reported two catalysts for lowtemperature methane activation to methanol: PtCl2(NH3)2 and PtCl2(bpym). It was shown that the ammine catalyst is much more active, but it decomposes rapidly in sulfuric acid to form a PtCl2 precipitate, while the bpym system is long-lived. To have a basis for developing new catalysts that would not decompose, we undertook a study of the structure, bonding, and stability of the PtCl2(NH3)2 and PtCl2(bpym) catalysts, using quantum mechanics (QM) [density functional theory (DFT) at the B3LYP/LACVP**(+) level] including solvation in sulfuric acid via the Poisson-Boltzmann continuum approximation. Critical results include the following: (1) The influence of a trans ligand Y on the Pt-X bond follows the order Cl->NH3(bpym)>OSO3H->0(empty site). Thus the Pt-N bond length is longer (up to 0.04 Å) and the Pt-N bond is weaker (up to 18 kcal/mol) when trans to a Cl-as compared to trans to OSO3H-. (2) The bpym ligand acts as both a ó-donor and a ð-acceptor. As bpym is protonated, the Pt-N bond strength decreases (by up to 51 kcal/mol). Thus, ¢H(soln, 453 K) for Pt(OSO3H)2(bpym) (69.6)> [Pt(OSO3H)2(bpymH)]+(49.4)>[Pt(OSO3H)2(bpymH2)]2+(18.7). (3) In sulfuric acid replacing the ammine ligands with bisulfate ligands is thermodynamically favorable [by△G(soln, 453 K)=-23 kcal/mol], whereas replacement of bpym with OSO3H- is unfavorable [by△G(soln, 453 K)=+16 kcal/mol]. (4) Replacement of chloride ligands with bisulfate ligands is thermodynamically unfavorable [by△G(soln, 453K)=～7kcal/mol for ammine and ～12 kcal/mol for bpym]. (5) Protonation of PtCl2(bpym) is thermodynamically favorable, leading to [PtCl2(bpymH)]+ as the stable species in sulfuric acid (by 8 kcal/mol). Thus we conclude that in hot concentrated sulfuric acid it is quite favorable for PtCl2(NH3)2 to lose its ammine ligands to form PtCl2, which in turn will dimerize and oligomerize, leading eventually to a (PtCl2)n precipitate and catalyst death. We find that PtCl2(bpym) is resistant to solvent attack, favoring retention of the bpym ligand in hot concentrated sulfuric acid. These results agree with experimental findings. The insights from these findings should help screen for stable new ligands in the design of new catalysts.

Based on density functional cluster model calculations, we present the first detailed mechanisms for the complete decomposition of NH3 to NHx(a) (x=0-2) on the Si(100)-(2