参考文献：
M. P. Kelley, J. Su, M. Urban, M. Luckey, E, R. Batista, P. Yang, and J. C. Shafer, On the Origin of Covalent Bonding in Heavy Actinides, J. Am. Chem. Soc., 2017, 139, 9901–9908 (2017).
在核燃料循环中，将高放射性次锕系元素从镧系和其他裂变产物中分离出来，对于使用过的核燃料回收再利用，以及核废物储存，都是至关重要的。通常通过锕系元素与配体之间的相互作用差异来进行分离。上世纪四十年代，学界已经热烈争论过锕系-配体之间的“共价”概念，假定次锕系元素形成了部分共价键，导致其5f轨道与配体轨道混合。
Top: average energy levels of molecular orbitals composed of An 6D (red), An 5f (blue), and oxygen 2p (grey) orbitals, weighted by MO composition. Energies of the 5f orbitals decrease across the series, becoming degenerate with the ligand orbitals at Bk. Right: this corresponds with a change in 5f molecular orbitals from highly localized (Am) to delocalized (Cf), representing significantly greater orbital mixing between the An 5f and ligand orbitals. Bottom: ΔΔG (relative to Am) for the formation of the 1:3 An:DPA complex; note the difference in both the experimental and DFT data between Cm and Bk.
著名的洛斯阿拉莫斯国家实验室使用ADF中的相对论密度泛函理论，研究了Am、Cm、Bk、Cf离子与DPA（dipicolinate）配体之间的成键。通过实验热力学测量，观察到Cm和Bk之间络合能的差异；并发现这个差异与锕系5f轨道能量降低到配体的配位氧原子2p轨道能级附近，以及5f轨道的离域性有关。分离过程常常有赖于络合能的这种微小差异，而能量的（近）简并导致共价的机制，可能为锕系分离方法设计带来新的策略。