The top panel of Fig. shows the FES we obtain for the Zr 4+ -DFO complex along the CN coordinate. As a visual guide for the reader, the solid red vertical line shows the magnitude of the CN obtained from our initial optimized structure. Also shown, are error bars in the FES, constructed using the Monte Carlo bootstrap method, as implemented in Ref. 29. Rather strikingly, we observe that the DFT-optimized structure does not correspond to the global free energy minimum, but is rather high up in energy indicating that, at room temperature, the six-fold coordination in DFO is highly unstable by at least 20k B T. The global minimum of the FES is observed at CN ∼3.5, meaning that there are three to four oxygen atoms of DFO coordinated to Zr 4+ .