Hydrogen adsorption at platinum electrodes was investigated by B3LYP quantum-chemical calculations. Electric-field effects were simulated either by charging the cluster models or by considering the cluster in the presence of a uniform external field. The results show that the large tuning rate of the Pt–H frequency should be attributed to the work function shift with the change of electrode potential, and the experimentally observed red-shift of the Pt-H frequency with decrease of electrode potential would originate from the lateral interaction of the Pt-H bonds.

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.

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

A series of CASSCF calculations were performed on the ground states of NiCO and FeCO. The contributions of the σπ interactions are checked by examining the validity of the CASSC calculation to describe the molecule with a particular choice of the active space. The calculation results substantiate that the stability of MCO is determined by a balance between π donation from the metal 3dπ to the CO2π and p repulsion between the metal σ electrons and the CO5σ lone pair and, at the same time, emphasizes the importance of the synergistic σπ interactions between the metal and the CO group. The relative importance of σπ interactions depends on the nature of the metal. In the case of NiCO, it is the π donation from Ni 3dπ to CO2π that makes the σ largest contribution to the formation of the Ni-CO bond, while in the case of FeCO, it is the correlation of σ electrons that holds the metal and CO together.

Three principles, namely, a neutrality princi-ple, a stoichiometry principle, and a coordination principle are proposed as criteria for building up cluster models of metal oxides. Particular attention is focused on how to cut out a stoichiometric cluster which possesses the smallest boundary e