  Context. Molecular Clouds (MCs) are the place where stars are formed and
their feedback starts to take place, regulating the evolution of galaxies.
Therefore, MCs represent the critical scale at which to study how ultra-violet
(UV) photons emitted by young stars are reprocessed in the far-infrared (FIR)
by interaction with dust grains, thereby determining the multi-wavelength
continuum emission of galaxies. Aims. Our goal is to analyze the UV and IR
emission of a MC at different stages of its evolution and relate its absorption
and emission properties with its morphology and star formation rate. Such a
study is fundamental to determine how the properties of MCs shape the emission
from entire galaxies. Method. We consider a radiation-hydrodynamic simulation
of a MC with self-consistent chemistry treatment. The MC has a mass $M_{\rm MC}
= 10^5 ~ M_\odot$, is resolved down to a scale of $0.06\, \rm pc$, and evolves
for $\simeq 2.4$~Myr after the onset of star formation. We post-process the
simulation via Monte Carlo radiative transfer calculations to compute the
detailed UV-to-FIR emission of the MC. Such results are compared with data from
physically-motivated analytical models, other simulations, and observations.
Results. We find that the simulated MC is globally UV-optically thick, but
optically-thin channels allow for photon escape ($0.1\%-10\%$), feature which
is not well-captured in the analytical models. Dust temperature spans a wide
range ($T_{\rm dust} \sim 20-300$~K) depending on the dust-to-stellar geometry,
which is reproduced reasonably well by analytical models. However, the
complexity of the dust temperature distribution is not captured in the
analytical models, as evidenced by the 10 K (20 K) difference in the mass
(luminosity) average temperature. Indeed, the total IR luminosity is the same
in all the models, but the IR emission -abridged
