The full manuscript for this work can be obtained from: [https://geochemicaltransactions.springeropen.com/articles/10.1186/s12932-017-0040-5][1] DFT optimization (including PBE96, PBE96+Grimme2, PBE0) and AIMD simulations (PBE96 and PBE96+Grimme2) were carried out for the isolated 100 Goethite surface and the 100 Goethite + water interface. The AIMD simulations were performed for a (3x2) surface slab 65 water molecules between the slabs (near 1g/cm3) at temperature near 300 K. The energetics of three different surface terminations from DFT PBE96 optimization calculations, in which the surface is neutral and Fe3+ cations maintain an octahedral coordination, were compared and it was found the lowest energy surface contained an exposed surface Fe3+ that was capped by a (weakly bound) water molecule and shared a hydroxide with a neighboring Fe3+. Finding termination to be the most stable agrees with prior MD and DFT calculations. This termination was also the largest fraction surface found in fitting CTR data by Ghose et al.. The other two surfaces tried, which were capped by two hydroxides, were found to be approximately 27 kJ/mol and 34 kJ/mol less stable per 1x1 surface slab (or 431 kJ/mol and 564 kJ/mol for the 3x2 surface slab). Solvating the slabs with 53 H2O molecules had very little effect on the relative energetics between the slabs, where the solvated 3x2 slabs capped by two hydroxides were found to have average energies that were 398 kJ/mol and 575 kJ/mol higher then the solvated slab is capped by a water molecule. It was found that each water molecule capping a surface Fe was very loosely bound for the lowest energy surface termination (Surface I). Similar results were observed in prior PBE96+U calculations by Kubicki et al., and suggest the surface Fe3+ is under-coordinated with only five neighboring oxygen atoms. This result was also observed with calculations using the dispersion corrected PBE96+Grimme2 and hybrid PBE0 exchange correlation functionals. The Fe-OH2 bond distances were found to be 2.45 Å, 2.39 Å, and 2.41 Å for the PBE96, PBE96+Grimme2, and PBE0 levels respectively. Solvating the slabs with 53 H2O molecules only contracted these distances slightly. The average Fe-OH2 bond distances in the AIMD simulations that were found to be 2.40 Å and 2.34 Å for the PBE96 and PBE96+Grimme2 theories respectively were still considerably larger than 2.15 Å seen in the model fitted from CTR data and the 2.09 Å -2.15 Å from prior published MD calculations by DeLeeuw, Kerisit, Boiley, Rustad, and others. The full AIMD simulations of 100 Goethite + water interface (Surface I cleavage) showed that the polarization of the water layer due to the surface (and vice versa) is fairly small and localized only in the immediate vicinity of the interface. There are 3 types of water molecules. The first type is the capping water molecule (OII) that is loosely bonded to the surface Fe3+ . The second type of water molecule (OIII) is hydrogen bonded to the surface hydroxyl, and the third type is bulk water. This classification is supported by several analysis including density difference plots, Wannier orbitals, and power spectra. While the OII and OIII water molecules form ordered water layers on surface they do not bond to the surface very strongly and as a result these water molecules exchanged readily with the bulk water molecules. Moreover because these water molecules are only weakly bonded, the surface Fe3+ are often not bonded above by the water molecules, which results in surface Fe that are only five-fold coordinated. To account for the large surface Fe-OH2 distances in the DFT calculations it was proposed that the surface Fe3+ atoms, which are already fully valent with only 5 neighbors. Bond valence theory calculations supported this assertion. If the under-coordinated surface Fe3+ atoms in DFT calculations is correct then other metal oxide water interfaces might also have under-coordinated surface cations, in particular oxide surfaces where a surface cation is capped by a water molecule such as the 100 plane of diaspore, and the R-planes of sapphire and hematite. Of course another possibility could be there is fundamental problem with current electronic structure methods for modeling these interfaces. Cleary this is an important issue and warrants further study. [1]: https://geochemicaltransactions.springeropen.com/articles/10.1186/s12932-017-0040-5