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tokens: ( Stimpfl et al. 2006 ). Model B (green curve) is constructed by matching the surface potential energy distribution shown in Stimpfl et al. (2006) . Here sites with binding energies smaller than 8420 K (70 kJ mol −1 ) are included, contrary to model A. Finally, model C (blue curve) includes 8% of surface sites with adsorption energies higher at 25 260 K (210 kJ mol −1 ), which were not taken into account in model A although claimed by the atomistic model of Stimpfl et al. (2006) . The surface energy potential distribution of the adsorption sites of the crystal surface determines the coverage of the adsorbed water molecules. As shown in the right plot of Fig. B.1 , in model C the surface coverage is 5% higher than in model A at 700 K and n H 2 O = 6 × 10 10 cm −3 . The surface potential energy distribution of model B allows only half the coverage than seen in model A and C, due to the higher hydrophobicity of the surface. We can conclude that the choice of the adsorption energy of the sites does not explain the factor of three difference in surface coverage between our work and the Muralidharan et al. (2008) simulations. Fig. B.1. Surface potential energy distribution ( left plot ) and surface site density ( middle plot ) of three models: A (red curve), B (green curve), and C (blue curve). The surface coverage for the three models is calculated at 700 K and n H 2 O = 6 × 10 10 cm −3 ( right plot ). Fig. C.1. Effect of water cluster on surface coverage data. Left panel : surface coverage (H 2 O nm −2 ) as a function of time (s) at n H 2 O = 6 × 10 10 cm −3 and T = 700 K, with cluster effect (pink curve) and without (light blue curve). Right panel : snapshots of the grid resembling the [1 0 0] forsterite surface showing the adsorption energy of the occupied sites without (middle plot) and with (right plot) cluster effect. The color scale indicates the adsorption energy in the range from 13 000 K (blue) to 25 000 K (yellow). Appendix C: Water cluster simulation In our MC models the water molecules randomly “walk” on the surface until they adsorb at a favorable site. As two water molecules sit on neighboring sites, a hydrogen bond links the two species, and a dimer can be created with the total binding energy higher than the ones of single molecules. This generates the formation of water clusters for which the binding energy increases with increasing size ( González et al. 2007 ). In two dimensions, the binding energies increase with the number of water molecules present in the cluster, while for a 3D cluster Lin et al. (2005) show that the binding energy mainly depends on the number of water molecules close to the surface. In this work we used a simple way to calculate the binding energies, which increase linearly with the number of neighboring water molecules ( Dartois et al. 2013 ), but we limited the number of water molecules to four. Water clusters have been implemented in previous MC simulations to study the formation of water on carbon grains