  We present a computer program and underlying model to calculate the electric
susceptibility of a gas, which is essential to predict its absorptive and
dispersive properties. Our program focusses on alkali-metal vapours where we
use a matrix representation of the atomic Hamiltonian in the completely
uncoupled basis in order to calculate transition frequencies and strengths. The
program calculates various spectra for a weak-probe laser beam in an atomic
medium with an applied axial magnetic field. This allows many optical devices
to be designed, such as Faraday rotators/filters, optical isolators and
circular polarisation filters. Fitting routines are also provided with the
program which allows the user to perform optical metrology by fitting to
experimental data.
