  A comprehensive model for explaining scintillation yield in liquid xenon is
introduced. We unify various definitions of work function which abound in the
literature and incorporate all available data on electron recoil scintillation
yield. This results in a better understanding of electron recoil, and
facilitates an improved description of nuclear recoil. An incident gamma energy
range of O(1 keV) to O(1 MeV) and electric fields between 0 and O(10 kV/cm) are
incorporated into this heuristic model. We show results from a Geant4
implementation, but because the model has a few free parameters, implementation
in any simulation package should be simple. We use a quasi-empirical approach,
with an objective of improving detector calibrations and performance
verification. The model will aid in the design and optimization of future
detectors. This model is also easy to extend to other noble elements. In this
paper we lay the foundation for an exhaustive simulation code which we call
NEST (Noble Element Simulation Technique).
