For generating spectra, the VPL has two models available for use: a 1-D photochemical model, and a 1-D radiative-convective climate model. The photochemical model solves for chemical species/reactions, and calculates photolysis rates; while the climate model calculates fluxes and absorption. Both models are capable of modeling high CO2 atmospheres, and high O2 atmospheres. 
1-D Radiative Convective and Photochemical Models
For generating top of the atmosphere flux in the solar system (or for exoplanets around G2 stars), VPL uses a solar flux model which is applicable at any object and from the zero-age main sequence until 8 Gyr:
Solar Flux Model
For estimating the boundaries of the habitable zone around main-sequence stars (both in terms of distance and stellar flux), VPL offers an interactive calculator tool:
Habitable Zone Calculator