  The dynamical state of massive clumps is key to our understanding of the
formation of massive stars. In this work, we study the kinematic properties of
massive clumps using synthetic observations. We have previously compiled a very
large catalog of synthetic dust-continuum compact sources from our 250 pc,
SN-driven, star formation simulation. Here, we compute synthetic $\rm
N_{2}H^{+}$ line profiles for a subsample of those sources and compare their
properties with the observations and with those of the corresponding
three-dimensional (3D) clumps in the simulation. We find that the velocity
dispersion of the sources estimated from the $\rm N_{2}H^{+}$ line is a good
estimate of that of the 3D clumps, although its correlation with the source
size is weaker than the velocity-size correlation of the 3D clumps. The
relation between the mass of the 3D clumps, $M_{\rm main}$, and that of the
corresponding synthetic sources, $M_{\rm SED}$, has a large scatter and a slope
of 0.5, $M_{\rm main} \propto M_{\rm SED}^{0.5}$, due to uncertainties arising
from the observational band-merging procedure and from projection effects along
the line of sight. As a result, the virial parameters of the 3D clumps are not
correlated with the clump masses, even if a negative correlation is found for
the compact sources, and the virial parameter of the most massive sources may
significantly underestimate that of the associated clumps.
