  As liquid argon (LAr) detectors are made at progressively larger sizes,
accurate models of LAr optical properties become increasingly important for
simulating light transport, understanding signals, and developing analyses. The
refractive index, group velocity, and Rayleigh scattering length are
particularly important for vacuum ultraviolet (VUV) and visible photons in
detectors with diameters much greater than one meter. While optical
measurements in the VUV are sparse, recent measurements of the group velocity
of 128 nm photons in LAr provide valuable constraints on these parameters.
These calculations are further complicated by the dependence of optical
parameters on thermodynamic properties that might fluctuate or vary throughout
the argon volume. This manuscript presents the model used by DEAP-3600, a dark
matter direct detection experiment at SNOLAB using a 3.3 tonne LAr
scintillation counter. Existing data and thermodynamic models are synthesized
to estimate the wavelength-dependent refractive index, group velocity, and
Rayleigh scattering length within the detector, and parameters' uncertainties
are estimated. This model, along with in situ measurements of LAr scintillation
properties, is benchmarked against data collected in DEAP-3600, providing a
method for modeling optical properties in large LAr detectors and for
propagating their uncertainties through downstream simulations. Updates are
also presented of the Noble Element Simulation Technique (NEST) software,
widely used to model scintillation and ionization signals in argon- and
xenon-based detectors.
