The time series of the CN (upper panel of Fig. ) shows that there is a transition from CN ≃ 5.5 to CN ≃ 3.5 at ∼ 2ns. This transition is irreversible; the CN never reverts back to the original value for the rest of the simulation lasting approximately 200ns. The bottomright panel of Fig. represents the snapshot of the simulation at 200ns. The structure of the complex indicates a fewer number of DFO oxygen atoms involved in Zr 4+ complexation. The fact that the hydroxamate group closest to the NH 3 terminus has moved away from the inner Zr 4+ coordination sphere, suggests that, although the initial configuration (DFT-optimized structure) is likely to be the most stable structure on the potential energy landscape relevant at 0K, its stability is altered by both temperature and water. We turned to construct the free energy landscape of the Zr 4+ -DFO complex to better understand the results in Fig. where there are no events where the initial structure is re-visited on the timescale of hundreds of nanoseconds. Therefore, one cannot reliably construct the correct Boltzmann weighted probabilities of the configurations in order to determine relative free energies of different conformational states. To circumvent this problem, we focused on examining the free energy profile along the CN using the umbrella sampling technique. The principle behind this method is that one adds an external potential to the system in order to achieve a non-Boltzmann sampling of regions that are poorly explored, which can be subsequently re-weighted to determine the correct free energy of the system. In detail, we performed a series of 1µsec long MD simulations of the Zr 4+ -DFO complex in water, in which a harmonic biasing potential is put on the CN at different values. In total, we generated approximately 9µsec of simulations for our analysis. All the simulations have the same value of the harmonic constant (k = 20k B T ), but differ in the value of the center of the harmonic potential (centers go from CN = 2 to CN = 6, with a 0.5 step). This means that in each simulation, we are forcing (biasing) a specific number of coordinating hydroxamate oxygen atoms, given by the value of the center of the bias. The FES is then reconstructed from the probability distribution of the CN obtained from each of these biased simulations, using the WHAM method (code from Ref. 29).