FDMX, or Finite Difference Method for XAFS, is an integrated software package designed for use with the full-potential FDMNES package to calculate extended energy-range X-ray absorption fine structure (XAFS) as well as X-ray absorption near-edge structure (XANES). It incorporates several routines and a large array of tabulated data to extend the application of the basic framework of FDMNES to higher energy photoelectron absorption spectra in a computationally efficient manner. These extensions notably include a physical modelling of thermal motion and outer-shell absorption contributions, and a highly sophisticated self-consistent plasmon-coupling model of inelastic electron scattering, quantifying the lifetime broadening associated with the imaginary component of the solid-state electron self-energy. This approach allows accurate calculations of X-ray absorption spectra over an energy range spanning several hundred electronvolts from the pre-edge region to the smooth atom-like region within a single consistent framework. FDMX utilizes a high-density representation of the electronic potential, and thereby provides unique insights into problems requiring detailed knowledge of the energy-dependence of the XAFS spectrum, particularly for large molecular compounds, organic and organo­metallic materials. Examples of these insights, including experimental analysis of low-energy electron inelastic mean free paths and correlated thermal displacement parameters, are discussed.