Electron Pair Localization Function (EPLF) for Density Functional Theory and ab Initio Wave Function-Based Methods: A New Tool for Chemical Interpretation

We present a modified definition of the Electron Pair Localization Function (EPLF), initially defined within the framework of quantum Monte Carlo approaches [Scemama, A.; Caffarel, M.; Chaquin, P. J. Chem. Phys. 2004, 121, 1725] to be used in Density Functional Theories (DFT) and ab initio wave-function-based methods. This modified version of the EPLF while keeping the same physical and chemical contents is built to be analytically computable with standard wave functions or Kohn-Sham representations. It is illustrated that the EPLF defines a simple and powerful tool for chemical interpretation via selected applications including atomic and molecular closed-shell systems, sigma and pi bonds, radical and singlet open-shell systems, and molecules having a strong multiconfigurational character. Some applications of the EPLF are presented at various levels of theory and compared to Becke and Edgecombe's Electron Localization Function (ELF). Our open-source parallel software implementation of the EPLF opens the possibility of its use by a large community of chemists interested in the chemical interpretation of complex electronic structures.