  We present a new simulated galaxy cluster catalog based on the IllustrisTNG
simulation. We use the Mulguisin (MGS) algorithm to identify galaxy
overdensities. Our cluster identification differs from the previous FoF cluster
identification in two aspects; 1) we identify cluster halos based on the galaxy
subhalos instead of unobservable dark matter particles, and 2) we use the MGS
algorithm that separates galaxy overdensities hosted by massive galaxies. Our
approach provides a cluster catalog constructed similar to the observed cluster
catalogs using spectroscopic surveys. The MGS cluster catalog lists 303 halos
with M$_{200} > 10^{14}$ M$_{\odot}$, including $\sim 10\%$ more than the FoF.
The MGS catalog includes more systems because we separate some independent
massive MGS cluster halos that are bundled into a single FoF algorithm. These
independent MGS halos are apparently distinguishable in galaxy spatial
distribution and the phase-space diagram. Because we constructed a refined
cluster catalog that identifies local galaxy overdensities, we evaluate the
effect of MGS clusters on the evolution of galaxies better than using the FoF
cluster catalog. The MGS halo identification also enables effective
identifications of merging clusters by selecting systems with neighboring
galaxy overdensities. We thus highlight that the MGS cluster catalog is a
useful tool for studying clusters in cosmological simulations and for comparing
with the observed cluster samples.
