  The TNG300-1 run of the IllustrisTNG simulations includes 1697 clusters of
galaxies with $M_{200c}>10^{14}$M$_\odot$ covering the redshift range
$0.01-1.04$. We build mock spectroscopic redshift catalogues of simulated
galaxies within these clusters and apply the caustic technique to estimate the
cumulative cluster mass profiles. We compute the total true cumulative mass
profile from the 3D simulation data and calculate the ratio of caustic mass to
total 3D mass, $\mathcal{F}_\beta$, as a function of cluster-centric distance
and identify the radial range where $\mathcal{F}_\beta$ is roughly constant.
The filling factor, $\mathcal{F}_\beta=0.41\pm 0.08$, is constant on a plateau
that covers a wide cluster-centric distance range, $(0.6-4.2)R_{200c}$. This
calibration is insensitive to redshift. The calibrated caustic mass profiles
are unbiased, with an average uncertainty of $23\%$. At $R_{200c}$, the average
$M^C/M^{3D}=1.03\pm 0.22$; at $2R_{200c}$, the average $M^C/M^{3D}=1.02\pm
0.23$. Simulated galaxies are unbiased tracers of the mass distribution.
IllustrisTNG is a broad statistical platform for application of the caustic
technique to large samples of clusters with spectroscopic redshifts for
$\gtrsim 200$ members in each system. These observations will allow extensive
comparisons with weak lensing masses and will complement other techniques for
measuring the growth rate of structure in the universe.
