The laboratory synthesis of complexes to mimic the structure of the Mn₄Ca cluster in the oxygen-evolving complex (OEC) of photosystem II is a very challenging task to accomplish. The most encouraging breakthrough in this field was recently achieved with the synthesis of a Mn₄Ca complex (Zhang et al., 2015) that shows a very similar core structure to the OEC. On the basis of density functional calculations, the structure and the redox potentials of this Mn₄Ca complex in acetonitrile are obtained with very good agreement to experiments. A possible mechanism for water oxidation is more problematic. If only the thermodynamics is considered and assuming a standard state of 1 mol/L, it turns out that up to five water molecules can be inserted into the complex with only a small cost. This leads to a barrier for OO bond formation which is 22.8 kcal/mol with an applied potential of 1.3 V. However, a study of the kinetics for the insertion of the critical water bridge between Mn3 and Mn4 indicates that the barrier for that process is quite high with 24.6 kcal/mol. A model where this water is not inserted also led to a rather high barrier for OO bond formation with 31.7 kcal/mol with an applied potential of 1.3 V. However, the barrier decreases significantly to only 13.4 kcal/mol with an applied potential of 1.7 V. The different barrier originates from the different energetic penalty for the formation of the catalytic competent S₄ state. A major experimental problem discussed below, is the instability of the complex, which does not allow a high water concentration. The best calculated overall mechanism obtained is essentially the same as the leading suggestion for the OEC, where the critical OO bond formation takes place at the S₄ state (formally Mn₄^IV,IV,IV,V) via direct coupling of a Mn^IV-oxyl radical and a di-Mn bridging oxo group.

模拟光系统II的析氧复合物（OEC）中的Mn ₄ Ca团簇结构的复合物的实验室合成是一项非常艰巨的任务。最近，该领域最令人鼓舞的突破是合成了Mn ₄ Ca络合物（Zhang等，2015），该络合物的核心结构与OEC非常相似。根据密度泛函计算，该Mn ₄的结构和氧化还原电势获得的乙腈中的Ca络合物与实验非常吻合。水氧化的可能机理更加成问题。如果仅考虑热力学，并假设标准状态为1 mol / L，那么事实证明，只需花费很少的成本，即可将多达五个水分子插入到该配合物中。这导致形成O O键的势垒为22.8 kcal / mol，施加的电势为1.3V。但是，对在Mn3和Mn4之间插入临界水桥的动力学的研究表明，该过程的势垒24.6 kcal / mol是相当高的。没有插入这种水的模型也导致了对O的相当高的阻挡在施加1.3 V的电势时形成31.7 kcal / mol的O键。但是，在施加1.7 V的势垒时，势垒显着降低至仅13.4 kcal / mol。催化能级为S ₄状态。下文讨论的主要实验问题是配合物的不稳定性，不允许高水浓度。计算得出的最佳总体机理与针对OEC的主要建议基本相同，在OEC中，通过直接偶联Mn在S ₄状态（正式为Mn ₄ ^{IV，IV，IV，V}）形成关键的O O键。^IV-氧基和二-Mn桥接氧代基团。