EWALD METHODS FOR POLARIZABLE SURFACES WITH APPLICATION TO HYDROXYLATION AND HYDROGEN-BONDING ON THE (012)SURFACE AND (001)SURFACE OF ALPHA-FE2O3

We present a clear and rigorous derivation of the Ewald-like method fo r calculation of the electrostatic energy of the systems infinitely pe riodic in two dimensions and of finite size in the third dimension (sl abs). We have generalized this method originally developed by Rhee et al. (Phys. Rev. B 40 (1989) 36) to account for charge-dipole and dipol e-dipole interactions and therefore made it suitable for treatment of polarizable systems. This method has the advantage over exact methods of being significantly faster and therefore appropriate for large-scal e molecular dynamics simulations. However, it involves a Taylor expans ion which has to be demonstrated to be of sufficient order. The method was extensively benchmarked against the exact methods by Leckner and Parry. We found it necessary to increase the order of the multipole ex pansion from 4 (as in the original work by Rhee et al.) to 6. In this case the method is adequate for aspect ratios (thickness/shortest side length of the unit cell) less than or equal to 0.5. Molecular dynamic s simulations using the transferable/polarizable model by Rustad et al . were applied to study the surface relaxation of the nonhydroxylated, hydroxylated and solvated surfaces of alpha-Fe2O3 (hematite). We find that our nonhydroxylated structures and energies are in good agreemen t with previous LDA calculations on alpha-alumina by Manassidis et al. (Surf. Sci. 285 (1993) L517). Using the results of molecular dynamics simulations of solvated interfaces, we define end-member hydroxylated -hydrated states for the surfaces which are used in energy minimizatio n calculations. We find that hydration has a small effect on the surfa ce structure, but that hydroxylation has a significant effect. Our cal culations, both for gas-phase and solution-phase adsorption, predict a greater amount of hydroxylation for the alpha-Fe2O3 (012) surface tha n for the (001) surface. Our simulations also indicate the presence of four-fold coordinated iron ions on the (001) surface. (C) 1997 Elsevi er Science B.V.