  Grains are determinant for setting the chemical, physical, dynamical, and
radiative properties of all the media in which they are present. Their
influence depends on the grain composition, size, and geometrical structure
which vary throughout the dust lifecycle. Grain growth arises in dense media as
traced by an enhancement of the FIR emissivity and by cloudshine and coreshine.
We investigate the imprint of the grain characteristics on the dust unpolarised
optical properties from the visible to the FIR for isolated grains and
aggregates. Using optical constants for both carbonaceous and silicate
materials, we derive the absorption and scattering efficiencies, the asymmetry
factor of the phase function, the albedo, and the mass opacity, using either
Mie theory or DDA. We study the effects of the size, porosity and shape of the
grains, and of the monomers constituting the aggregates, on the optical
properties. For aggregates we study the influence of the number of monomers and
of mixing monomer sizes. Grain structure changes result in variations at all
wavelengths. Porosity, grain elongation, and aggregation all produce an
increase in the FIR opacity. The spectral dependence of this increase depends
on the nature of the material composing the grain: it is independent of the
wavelength for insulators but not for conductors. For aggregates, the FIR
increase does not depend on the monomer size and saturates for aggregates
containing 6 or more monomers. For silicates, the MIR spectral feature at 18
microns is more sensitive to the details of the grain structure than the 10
microns feature. This study provides a basis for understanding the range of
variations achievable as a result of varying the grain characteristics. It
emphasises the importance of considering the detailed grain structure in
determining the dust optical properties and of using exact methods.
