  To understand the most-recent experiment on room-temperature
superconductivity in carbonaceous sulfur hydride (CSH) systems under high
pressure, we have performed extensive stoichiometry and structure searches of
ternary CSH compounds using generic evolutionary algorithms. Judged from the
formation enthalpy of different CSH compounds, our studies conclude that
certain levels of carbon doping (~5%-6%) in sulfur hydride (H$_3$S) in its R3m
and phases gives rise to the most-stable structure (second to the H$_3$S
itself) among the various CSH systems found. The replacement of a small amount
of sulfur atoms by carbon in compounds like C$_1$S$_{15}$H$_{48}$ and
C$_1$S$_{17}$H$_{54}$ results in a stronger electron-phonon coupling and a
higher averaged phonon frequency that increases with pressure, thereby leading
to room-temperature superconductivity at ~270 GPa. The calculated
superconducting transition temperature T$_c$ of C$_1$S$_{15}$H$_{48}$ and
C$_1$S$_{17}$H$_{54}$ as a function of pressure shows reasonably good agreement
with experimental measurements. Before transition to superconducting states at
~80 GPa, the CSH system is predicted to have a stoichiometry of
C$_2$S$_2$H$_{10}$ with a stable structure of P1 symmetry, which is supported
by the direct comparison of its Raman spectrum with experiment.
