  We use density functional theory (DFT) to explore the physical properties of
an $Er_{ W}$ point defect in monolayer $WS_{ 2}$. Our calculations indicate
that electrons localize at the dangling bonds associated with a tungsten
vacancy ($V_{W}$) and at the $Er^{ 3+}$ ion site, even in the presence of a net
negative charge in the supercell. The system features a set of intra-gap defect
states, some of which are reminiscent of those present in isolated $Er^{ 3+}$
ions. In both instances, the level of hybridization is low, i.e., orbitals show
either strong Er or W character. Through the calculation of the absorption
spectrum as a function of wavelength, we identify a broad set of transitions,
including one possibly consistent with the $Er^{ 3+}$ $4I_{ 15/2} \rightarrow
4I_{ 13/2}$ observed in other hosts. Combined with the low native concentration
of spin-active nuclei as well as the two-dimensional nature of the host, these
properties reveal $Er:WS_{ 2}$ as a potential platform for realizing spin
qubits that can be subsequently integrated with other nanoscale optoelectronic
devices.
