  Recent experimental study by Dias {\it et al.} claims to have discovered
room-temperature superconductivity in lutetium-nitrogen-hydrogen system at 1
GPa [Nature 615, 244 (2023)], which sheds light on the long-held dream of
ambient superconductivity. However, all follow-up experiments found no evidence
of superconductivity. The compositions and the crystal structures of the
lutetium-nitrogen-hydrogen system remain unknown. By employing the density
functional theory based structure prediction algorithm, we suggest that in
lutetium-nitrogen-hydrogen the major component is LuH$_2$ (Fm$\bar{3}$m),
together with minor LuN (Fm$\bar{3}$m). The blue LuH$_2$ at ambient pressure
will turn into purple and red color at higher pressures, possibly accompanied
by the formation of vacancies at hydrogen-sites. In LuH$_2$ and LuN, the
density of states at the Fermi level is dominated by the Lu-5d orbitals, while
those from hydrogen and nitrogen are very small, leading to the absence of
superconductivity in these two compounds. Nitrogen-doping to LuH$_2$ fails to
enhance the superconductivity as well. In this work, we identify the leading
components in N-doped lutetium hydride, explain its intriguing color changes
under pressure, and elucidate why superconductivity is absent in the follow-up
experiments.
