Chemists are familiar with the range of length-scales that are present in molecular electronic wave functions. There is a sharp cusp at the nucleus; atoms have core and valence regions;
molecules also possess inter-atomic bonding regions; and both atoms and molecules have a long exponential tail. By capturing the essence of molecules as being composed of perturbed atoms, the linear combination of atomic orbitals (LCAO) approximation has proven tremendously
successful. Part of this success stems from effective management of the length scales present in the problem. The atomic orbital basis functions naturally incorporate both the short- and long-range behavior. However, the non-local nature of atomic wave functions, especially in high-precision calculations, gives rise to inefficiencies, non-physical density matrices, and numerical problems due to severe linear dependence. Furthermore, the atomic orbital basis sets must be carefully designed and calibrated [1]. If used out of context (e.g., in charge-fitting [2] or resolution-of-the-identity methods [3,4]) uncontrolled errors can be introduced.