  Particle shape and aggregation have a strong influence on the spectral
profiles of infrared phonon bands of solid dust grains. Calculating these
effects is difficult due to the often extreme refractive index values in these
bands. In this paper, we use the Discrete Dipole Approximation (DDA) and the
T-matrix method to compute the absorption band profiles for simple clusters of
touching spherical grains. We invest reasonable amounts of computation time in
order to reach high dipole grid resolutions and take high multi-polar orders
into account, respectively. The infrared phonon bands of three different
refractory materials of astrophysical relevance are considered - Silicon
Carbide (SiC), Wustite (FeO) and Silicon Dioxide (SiO2). We demonstrate that
even though these materials display a range of material properties and
therefore different strengths of the surface resonances, a complete convergence
is obtained with none of the approaches. For the DDA, we find a strong
dependence of the calculated band profiles on the exact dipole distribution
within the aggregates, especially in the vicinity of the contact points between
their spherical constituents. By applying a recently developed method to
separate the material optical constants from the geometrical parameters in the
DDA approach, we are able to demonstrate that the most critical material
properties are those where the real part of the refractive index is much
smaller than unity.
