Molecular, Fe-catalyzed ethylene oligomerization provides access to a range of linear α-olefins (LAOs) that are used to produce polyethylene, lubricants, surfactants, and other commercial products. This work provides an experimental example of an Fe pendant donor diimine ((PDD)­Fe) catalyzed ethylene oligomerization that showcases very high olefin oligomer purity without branching and provides K (propagation/(propagation + termination)) values of LAOs fractions, which show larger K values as a function of carbon chain length. This experimental example provided an anchor point to try to identify a practical density functional theory (DFT) protocol to model ethylene oligomerization branching, propagation/termination, and K values. Using M06-L DFT calculations, we successfully modeled the very high oligomerization purity for the (PDD)Fe catalyst, compared to the lower purity for the Fe tridentate pyridine bisimine (PBI)Fe catalyst, which showed enhanced regioselectivity for migratory insertion between Fe–H intermediates and LAOs. Modeling propagation/termination and K values were significantly more challenging with most oxidation and spin states incorrectly predicting a significant preference for oligomerization termination. Therefore, caution should be used when trying to model these types of quantitative catalysis values.