With this atomic gradient coordinate system, we can conceptually perform a gradient bundle decomposition by first placing a sphere 𝑆 ! of radius 𝑑𝑟 (in practice, 𝑑𝑟 ≲ 0.2Å) centered at nuclear CP 𝑖. We then partition the surface of 𝑆 ! into differential area elements 𝑑𝐴 = 𝑑𝑟 " 𝑑𝜃 𝑑𝜙 (see Figure ). The Gs intersecting 𝑆 ! interior to a particular area element constitute a differential volume element, bounded by local zero-flux surfaces and called a differential gradient bundle, 𝑑GB ! (𝜃, 𝜙). 4 Importantly, the cross-sectional area of each 𝑑GB ! itself varies with arclength, i.e., 𝑑𝐴 = 𝑑𝐴(𝑠). The union of all 𝑑GB ! recovers the atomic basin. Notably, 𝑑GB ! are the smallest structures bounded by zero-flux surfaces, hence the smallest structures possessing well-defined energies and energy-mediated properties. The unambiguous regional electronic kinetic energy of zero-flux surface bounded regions applies only to the Laplacian family of kinetic energy operators. When operators outside this family are considered, there seems to be some ambiguity to regional kinetic energy regardless of how one partitions the system, so a "well-defined" kinetic energy may be unobtainable in an absolute sense .