Parametrization of a classical force field for iron oxyhydroxide/water interfaces based on Density Functional Theory calculations

Abstract

We perform a study of the bulk and surface properties of FeOx(OH)y phases in contact with water by means of electronic structure calculations based on Density Functional Theory (DFT), and develop a classical potential to study FeOx(OH)y/water interfaces in combination with standard biomolecular force fields. We identify the optimal position of the H atoms in the bulk unit cell of ferrihydrite and confirm an experimental prediction suggesting an antiferromagnetic ordering of the Fe(III) ions in the ground state. Surface free energy calculations reveal that the most stable (0001) ferrihydrite surface in equilibrium with bulk water and dissolved oxygen is a fully hydroxylated plane terminating octahedrally-coordinated Fe atoms. DFT potential energy profiles of small molecules (water, ammonia, formic acid) adsorbing at hydrated goethite and ferrihydrite surfaces are used as a basis to optimize and validate the parameters of the classical potential. The identified set of parameters is able to reproduce the DFT energy values at the surface/molecule equilibrium adsorption position, and predicts the heat of immersion of goethite and ferrihydrite surfaces in good agreement with experimental measurements for FeOOH particles. However, as a general tendency the equilibrium surface–molecules Fe–O bond distances are slightly underestimated, motivating future analyses of the adsorption free energy and adhesion forces of small peptides on ferrihydrite and goethite surfaces to further evaluate the predictive power of the developed force field.