  In this work we explore the phase diagram of the binary Nb-S system from
ambient pressures up to 250 GPa using $ab$ $initio$ evolutionary crystal
structure prediction. We find several new stable compositions and phases,
especially in the high-pressure regime, and investigate their electronic,
vibrational, and superconducting properties. Our calculations show that all
materials, besides the low-pressure phases of pure sulfur, are metals with low
electron-phonon coupling strengths and critical superconducting temperatures
below 15 K. Furthermore, we investigate the effects of phonon anharmonicity on
lattice dynamics, electron-phonon interactions, and superconductivity for the
novel high-pressure phase of $\mathrm{Nb}_2\mathrm{S}$, demonstrating that the
inclusion of anharmonicity stabilizes the lattice and enhances the
electron-phonon interaction.
