The effect of the size and shape of silver nanoparticles on their optical absorption properties is theoretically investigated to understand the plasmonic properties of these systems. Time-dependent density functional theory (TDDFT) calculations are employed to calculate the optical absorption spectra for a series of silver clusters (Ag n , n = 6−85) in various charge states whose structures are octahedral, truncated octahedral, and icosahedral. Octahedral Ag n clusters with n = 6, 19, 44, 85, truncated octahedral Ag n clusters with n = 13, 38, 55, 79, and icosahedral Ag n clusters with n = 13, 43, 55 are calculated. Charged systems are considered to obtain closed shell electronic structures. These calculations are performed with the ADF code with the BP86/DZ level of theory in the optimizations and the SAOP functional and LB94 functional in the excitation calculations. A sharp excitation peak originates from a mixture of orbital transitions, and a broad excitation arises from multiple excited states in octahedral, truncated octahedral, and icosahedral Ag n clusters. We predict that the absorption peak maximum red shifts as the cluster becomes larger and blue shifts as the shape of clusters changed from octahedral to icosahedral.

Evaluation and reparameterization of previously reported ReaxFF parameters (Järvi, T. T.; et al. J. Phys. Chem. A 2011, 115, 10315-10322) is carried out for Au-S-C-H systems. Changes in Au-S and Au-Au bond parameters and S-Au-S angle bending parameters yield improvements for bond bending potential energy surfaces. The new ReaxFF parameters lead to good agreement with density functional theory geometries of small clusters and gold-thiolate nanoparticles. The energies of Au38(SCH3)24 clusters are compared, and the new ReaxFF calculations are also in good agreement with PBE calculations for the isomer orderings. In addition, the relative energies of Au40(SCH3)24 nanoparticles and Au-thiolate SAMs are calculated using the updated parameters. These new ReaxFF parameters will enable the study of the geometries and reactivity of larger gold-thiolate nanoparticles.

The absorption spectra for dimers of Ag(4)(+2) and Ag(8) clusters at various interparticle distances are examined using time-dependent density functional theory (TDDFT) and configuration interaction singles (CIS) calculations. With TDDFT calculations employing the SAOP functional, minor peaks for Ag(4)(+2) and Ag(8) dimers appear as the interparticle distance decreases; these peaks are suggested to be charge transfer artifacts on the basis of CIS and TDDFT (CAM-B3LYP) calculations. The relationship of the absorption peak locations to the distance and orientation between T(d) Ag(20) dimers is also investigated. TDDFT calculations using the SAOP functional are used to determine excitation absorption spectra for eight different orientations of Ag(20) dimers. Although the Ag(20)T(d) monomer has a sharp peak, each dimer absorption spectrum is split due to lower symmetry. This splitting increases as the center of mass distance decreases. As the interparticle distance between the monomers decreases, the initially strong peaks decrease in intensity and red or blue shift depending on symmetry, while the minor peaks increase in intensity and red shift.

Ab initio simulations and calculations were used to study the structures and stabilities of copper oxide clusters, CunOn (n=1-8). The lowest energy structures of neutral and charged copper oxide clusters were determined using primarily the B3LYP/LANL2DZ model chemistry. For n ≥ 4, the clusters are nonplanar. Selected electronic properties including atomization energies, ionization energies, electron affinities, and Bader charges were calculated and examined as a function of n.