A molecularly detailed simulation method, designed to be efficacious for modeling conduction properties of closed shell atoms or molecules in solids, liquids, and at interfaces, has been developed. This approach successfully predicts the effective mass of a conduction electron in both solid xenon, and liquid xenon over a wide density range, as compared to experimental results. To model the electron-atom interaction, angular momentum and density-dependent semi-local pseudopotentials are employed. The pseudopotentials are first fit to reproduce the gas phase electron-xenon scattering phase shifts, and are then corrected to include many-body polarization effects in a reliable mean field fashion. The effective mass of a conduction electron is calculated by solving the Schrodinger-Bloch equation using Lanczos grid methods to obtain the Bloch wave vector (k) dependent energies in both the solid and the liquid. In the liquid phase, a representative sample of the fluid is replicated to form the "periodic" infinite system. This approximation is shown to be reliable as the effective mass does not depend on the system size or the particular configuration which is chosen. It is shown that the l=0 scattering in the condensed phase determines the k=0 ground state energies; these are coincident with the conduction band energy in this system. In contrast, the l=1 scattering is shown to determine the effective mass of the conduction electrons.