Periodic surface models were built from one bulk unit cell, with the vacuum gap between successive layers made equal to the length of the surface slab in the direction normal to the (1 0 0) plane.
This cell contains a 3 × 2 (100) surface slab with approximately 10 Å between slabs.
The other two surfaces examined, which were capped by two hydroxides, were found to be approximately 27 and 34 kJ/mol less stable per 1 × 1 surface slab (or 431 and 564 kJ/mol for the 3 × 2 surface slab).
The simulations were performed for a (3 × 2) surface slab containing 65 water molecules between the slabs (near 1 g/cm3) at temperature of 300 K.
The first anomaly showed a chaotic pattern in the GPR signal and appeared right beneath the existing road surface slab and was 3 4 m wide.
Except for the Fe OII distance, the bond distances and angles for the surface slab were in in good agreement with the fitted model from CTR experiments [22].
However, as noted above, the H-bond distances were significantly lengthened when the model was expanded to a 3 × 3 surface slab.
The goethite surface slab contained 24 Fe, 48 O and 60 H atoms, and the thickness of the slab was 9.4 Å (distance in a direction between two oxygen atoms on surface).
The solvated 3 × 2 slabs capped by two hydroxides (surface II and surface IID in Figs. 3 and 4) were found to have average energies that were 398 and 575 kJ/mol higher then the solvated Surface I slab (3 × 2 surface slab, Fig. 2).
Thus, the drilling confirmed that there was a void right under the road surface slab and the depth of this void was 0.76 m. Figure 7 shows the GPR data for this void correlated with the drilling result.
Vibrational frequencies were calculated for the energy-minimized bulk and surface slab models, using a finite difference method (i.e., each atom is displaced individually around its equilibrium position by approximately 0.1 Å) and numerical solution of the Hessian matrix.
