Above X denotes 3N coordinates and spin, and let x denote 3-dimensional coordinates and spin, and let x denote the Cartesian direction. What is the structure of Ψ? As introduced by Slater and Fock in the 1920s, recognizing the Fermionic character of electrons, many-body electronic-wavefunctions must be wholly anti-symmetric under particle-swapping. This is done by the n-electron Slater Determinant of 1-electron wavefunctions ψ: