  The LUX-ZEPLIN dark matter search aims to achieve a sensitivity to the
WIMP-nucleon spin-independent cross-section down to (1--2)$\times10^{-12}$\,pb
at a WIMP mass of 40 GeV/$c^2$. This paper describes the simulations framework
that, along with radioactivity measurements, was used to support this
projection, and also to provide mock data for validating reconstruction and
analysis software. Of particular note are the event generators, which allow us
to model the background radiation, and the detector response physics used in
the production of raw signals, which can be converted into digitized waveforms
similar to data from the operational detector. Inclusion of the detector
response allows us to process simulated data using the same analysis routines
as developed to process the experimental data.
