  We obtain the interaction potential for NaCs by fitting to experiments on
ultracold scattering and spectroscopy in optical tweezers. The central region
of the potential has been accurately determined from Fourier-Transform
spectroscopy at higher temperatures, so we focus on adjusting the long-range
and short-range parts. We use coupled-channel calculations of binding energies
and wave functions to understand the nature of the molecular states observed in
ultracold spectroscopy, and of the state that causes the Feshbach resonance
used to create ultracold NaCs molecules. We elucidate the relationships between
the experimental quantities and features of the interaction potential. We
establish the combinations of experimental quantities that determine particular
features of the potential. We find that the long-range dispersion coefficient
$C_6$ must be increased by about 0.9% to 3256(1) $E_\textrm{h} a_0^6$ to fit
the experimental results. We use coupled-channel calculations on the final
potential to predict bound-state energies and resonance positions.
