For example, various protonation states have been suggested for key residues surrounding the active site of the FeMoco unit, resulting in a cross-over between N2 absorption on Fe2 and Fe6 (Figure ). Remarkably, Bjornsson and co-workers have argued that some of the discrepancies in the computed protonation states may be caused by the choice of the exchange-correlation functional, with one protonation state being seemingly preferred when using one, while becoming unstable when another functional is applied. Complicating matters even further, computational evidence has been put forward that these protonation states may change over the course of the reaction mechanism. Other points of contention include the regiochemistry of the H + /e -sequential additions to the FeMoco unit that precede the N2-coordination. More specifically, for the quadruply reduced FeMoco, both "one-side" structures-in which all hydrogens are located on the active facet of the cofactor-and "two-side" structures-the four hydrogens are distributed across two facets-have been characterized computationally (Figure ). The former structure results in the formation of H2S (which can either remain bound or eventually leave the active site) and H2, and exposure of both Fe2 and Fe6, prior to the N2-absorption step, whereas the latter results in the formation of a thiolate (-SH), covalently bound to one of the Fe centers in the facet, as well as bridging hydrogen (i.e. Fe2-µH-Fe6).