  Constraints on the possible shape and clustering, as well as optical
properties, of grains responsible for the 2175 angstrom interstellar extinction
feature (interstellar UV bump) are discussed. These constraints are based on
the observation that the peak position of the interstellar UV feature is very
stable (variations <~1%), that the large variations in width (<~25%) are
uncorrelated with the peak position except for the widest bumps, and that the
shape of the feature is described extremely well by a Drude profile. The UV
extinction of small graphite grains is computed for various clustering models
involving Rayleigh spheres. It is shown that compact clusters qualitatively
satisfy the above observational constraints, except that the peak position
falls at the wrong wavelength. As an alternative to graphite to model the
optical properties of the interstellar UV feature carrier, a single-Lorentz
oscillator model is considered, in conjunction with a clustering model based on
clusters of spheres. Intrinsic changes in the peak position and width are
attributed to variations in chemical composition of the grains, impacting upon
the parameters of the Lorentz oscillator. Further broadening is attributed to
clustering. These models are shown to satisfy the above observational
constraints. Furthermore, the correlated shift of peak position with increased
width, observed for the widest interstellar UV features, is reproduced. Models
involving a second Lorentz oscillator to reproduce the FUV rise are also
considered. The impact of this extra Lorentz oscillator on the peak position,
width, and shape of the bump is investigated. Synthetic extinction curves are
generated to model actual ones exhibiting a wide range of FUV curvatures.
Physical mechanisms which might be of relevance to explain the variations of
these optical properties are discussed.
