The magnetic susceptibility of H2O, NH3, CH4, and H2O2 molecules has been evaluated by the perturbed HF scheme. Several wavefunctions, all of LCAO SCF MO type, have been considered. By using a double perturbation technique we derive some useful quantities which allow one to evaluate diamagnetic and paramagnetic contributions to the susceptibility for any gauge. The results corresponding to the largest basis sets are in satisfactory agreement with experimental data. The dependence of the evaluated quantities upon the gauge transformation corresponding to a change of the origin of the vector potential has been examined in detail.

The construction of the exact quantum propagator of a particle subjected to a constant force offers a challenge to existing propagator methodology. Here, besides considering the traditional approach, which involves an expansion in the complete set of energy eigenstates, we illustrate (i) the merits of a rather more elegant procedure founded on the ‘‘disentanglement’’ of the time-evolution operator and (ii) the power of a general treatment based on a time-dependent variational approach.

We have studied theoretically the far- and near-field scattering response of bimetallic Ag/Au core-shell and alloy nanoparticles. Particular emphasis is put on the near-field study, which is known to play a fundamental role in surface enhanced spectroscopies. The comparison between the scattering spectra of core-shell and alloy particles shows that for particles with a Au/Ag volume ratio greater than 2, the structural difference does not imply any significant difference in the optical response. For such particles, while the retardation effects are not negligible, the scattering at the interface between the two metals in the core-shell case does not seem to modify the scattering behavior. The scattering at the interface is conversely not negligible for particles with a lower Au/Ag ratio, where the particle inner structure seems to be important.

Near-field and far-field extinction and scattering efficiencies for UV−vis-irradiated spherical gold nanoparticles of varying size have been estimated as a function of the radiation wavelength. Several different dielectric function models are used to explore the role of quantum-size effects. The results suggest that the utilization of bulk dielectric data does not capture quantitatively important size effects and may correspond to an unjustified oversimplification.

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