  The size and composition of dust grains are critical in setting the
dynamical, physical and chemical evolution of the media in which they are
present. Thanks to facilities such as ALMA and in the future the SKA, their
thermal emission in the (sub)mm to cm has become a convenient way to trace
grain properties. Our aim is to understand the influence of the grain
composition and size distribution on the shape of their SED in dense ISM
regions such as molecular clouds, prestellar cores, YSOs and protoplanetary
discs. Starting from the optical constants defined in the THEMIS model for
amorphous hydrogenated carbon and silicate grains in addition to water ice, we
define 6 material mixtures representative of the expected dust composition in
dense ISM regions. The optical properties of 0.01 micron to 10 cm grains are
then calculated with effective medium and Mie theories. The corresponding SEDs
are calculated for isolated clouds either externally heated by the ISRF alone
or in addition to an internal source. The 3 main outcomes of this study are as
follows. First, the dust mass absorption coefficient strongly depends on both
its composition and size distribution potentially leading to errors in dust
mass estimates by factors up to 3 and 20, respectively. Second, it appears
almost impossible to retrieve the grain composition from the (sub)mm to cm SED
shape alone as its spectral index for lambda > 3 mm does not depend on
composition. Third, using the true dust opacity spectral index to estimate
grain sizes may lead to erroneous findings as the observed spectral index can
be highly modified by the dust temperature distribution along the
line-of-sight, which depends on the specific heating source and on the geometry
of the studied region. Based on the interpretation of only the spectral shape
of (sub)mm to cm SEDs, the determination of the dust masses, compositions and
sizes are highly uncertain.
