  A grid of 20 millions 3-1100$\mu$m SED models is presented for synthetic
young clusters embedded in dense clumps. The models depend on four primary
parameters: the clump mass M$_{clump}$ and dust temperature T$_{dust}$, the
fraction of mass f$_{core}$ locked in dense cores, and the age of the clump
t$_{SF}$. We populate the YSO clusters using the IMF from Kroupa(2001) and the
YSOs SED models grid of Robitaille et al. (2006). We conduct extensive testing
of SED fitting using a simulated dataset and we find that M$_{clump}$
essentially depends on the submillimeter portion of the SED, while T$_{dust}$
is mostly determined from the shape of the SED in the 70-350$\mu$m range.
Thanks to the large number of models computed we verify that the combined
analysis of L/M, [8-24] and [24-70] colours removes much of the SEDs
f$_{core}$-t$_{SF}$ degeneracy. The L/M values are particularly useful to
diagnose f$_{core}$. L/M$\leq$1 identifies protoclusters with f$_{core}\leq$0.1
and t$_{SF} \leq 10^5$ years, while L/M$\geq$10 excludes f$_{core}\leq$0.1. We
characterize lower limits of L/M where ZAMS stars are not found in models, and
we also find models with L/M $\geq$10 and no ZAMS stars, in which
[8-24]$\geq0.8\pm 0.1$ independently from M$_{clump}$, temperature and
luminosity. This is the first set of synthesis SED models suited to model for
embedded and unresolved clusters of YSOs. A set of new evolutionary tracks in
the L/M diagram is also presented.
