  We consider the effect of Au nanospheres of subwavelength sizes on the decay
and energy transfer rates of quantum systems placed in the proximity of these
nanospheres. We find that, for the sphere sizes considered in this
contribution, the radiative decay rate is barely affected by the presence of
the nanosphere, whereas the non-radiative decay rate is greatly enhanced due to
energy transfer from the quantum system to the nanosphere, leading to a strong
quenching of the emission of the quantum system. The emission wavelength of the
quantum emitter and its intrinsic quantum yield play an important role and the
impact of both has to be considered together when investigating their effect on
the non-radiative decay rate. The energy transfer process from the emitter to
the nanosphere has a complicated distance dependence, with a 1/r^6 regime,
characteristic of the F\"orster energy transfer mechanism, but also exhibiting
other distance dependence regimes. In the case of a donor-acceptor pair in the
presence of a Au nanosphere, the donor couples strongly to the nanosphere,
acting as an enhanced dipole; the donor-acceptor energy transfer rate then
follows a F\"orster trend, with an increased F\"orster radius. The angular
dependence of the energy transfer efficiency between donor and acceptor has a
strong dipole-dipole trend for small spheres and deviates from it for larger
spheres, especially when the donor and acceptor are on opposite sides of the
sphere. The spectral overlap of the donor emission, acceptor absorption and
gold nanosphere extinction shows an interesting trend in that the largest
F\"orster radius is obtained when the donor emission and acceptor absorption
maxima are somewhat red-shifted from the localized surface plasmon peak in the
extinction spectrum of the Au nanosphere, being located between it and the
near-field scattering maximum.
