A generic method for producing potentials to model organic-mineral systems is proposed. The method uses existing potentials for the components of the system and produces cross-term potentials between these components. The existing potentials are fitted to known mineral structures modeled with charges that mimic the Coulombic potential at the organic-mineral interface. The method has been applied to supply a set of potentials to model calcite biomineralization, including water-calcite, bicarbonate ions, and a set of organic functional groups with calcite. Tests comparing the results from ab initio and other potential-based calculations demonstrate that the new potential set is reliable and accurate.

Density functional theory calculations were employed to explore the interaction between water and the {1014} surface of calcite. In addition a defective {1014} surface and stepped surfaces in contact with water were investigated. A series of percentage water coverages and water configurations were explored, including dissociated water states. Static relaxations found associated water to be favourable on the {1014} surface, although a metastable dissociated state 1.77eV higher in energy was found.Molecular dynamics (MD) simulations of low water coverage reveal fluctuations in the H-O water bond when the H atom is directed towards a surface CO 3 ion. Desorption of an H 2 O molecule was observed in simulations above 900K. Water was found to be strongly bound to the perfect {1014} surface, with an adsorption energy of -0.91eV. MD simulations of a defective {1014} surface found water to favour dissociation at CO 3 vacancies. However, water at Ca vacancies diffused across the surface to form a bond with the nearest surface Ca ion. Water was also found to favour an associated state at both acute and obtuse steps. On all these imperfect surfaces water was found to adsorb strongly to the surface, with adsorption energies ranging from -0.99eV to -1.60eV.

The interaction between water and calcite surfaces is relevant to a broad range of technological processes, but a fundamental understanding of the nature of the adsorbed water is still lacking. In an earlier publication we used density functional theory calculations to calculate the interaction between water and perfect (101 j 4) calcite surfaces. Water was found to be strongly adsorbed as associated molecules. In this paper water adsorption on (101 j 4) calcite surfaces with steps and vacancies is investigated. A water molecule was found to bind more strongly to acute steps than to obtuse steps. The lowest energy position was found to be the base of the step for acute steps and on top of the step for obtuse steps. Water molecules were found to exhibit very strong binding to surface vacancies. Associative adsorption was favored near cation vacancies; however, the water was found to dissociate, to form a bicarbonate ion and a hydroxide ion, near anion vacancies.