FDMNES

FDMNES stands for Finite Difference Method Near Edge Structure. It is a computational technique used in X-ray spectroscopy to simulate X-ray absorption spectra (XAS), particularly in the near-edge region. This region, also known as the X-ray Absorption Near Edge Structure (XANES) or Near Edge X-ray Absorption Fine Structure (NEXAFS), is characterized by sharp features related to the electronic transitions from a core level to unoccupied electronic states.

The FDMNES method solves the Schrödinger equation for an electron interacting with a core hole potential created by the absorption of an X-ray photon. Unlike other approaches such as muffin-tin-based methods, FDMNES employs a real-space finite difference scheme to represent the electron wavefunction and the potential. This allows for the accurate treatment of complex, non-spherical potentials and disordered systems.

Key characteristics of FDMNES calculations include:

• Real-Space Approach: Solves the Schrödinger equation directly in real space, without requiring any specific shape approximation of the atomic potential.
• Finite Difference Method: Discretizes space into a grid and approximates derivatives using finite differences. This allows for flexibility in the geometry and potential.
• Full Multiple Scattering: Incorporates the full multiple scattering of the photoelectron, accounting for interactions with all neighboring atoms.
• Versatility: Applicable to a wide range of materials, including crystals, amorphous solids, and molecules.
• Core-Hole Effects: Typically includes the effect of the core hole created during X-ray absorption, which can significantly influence the spectrum.

The output of an FDMNES calculation is an XAS spectrum, which can be compared with experimental data to gain information about the electronic structure, local atomic environment, and chemical state of the absorbing atom. The analysis of XAS spectra using FDMNES simulations is a powerful tool in various fields, including materials science, chemistry, and condensed matter physics. It is used to study the electronic structure of materials, determine the local atomic environment around specific elements, and investigate the effects of chemical bonding.