An ab initio and classical molecular dynamics investigation of the structural and vibrational properties of talc and pyrophyllite

The structural and vibrational properties of two uncharged 2:1 phyllosilicates, talc and pyrophyllite, are investigated via ab initio and classical molecular dynamics simulations. The quantum mechanical simulations are based on plane-wave pseudopotential density functional theory (DFT), which is shown to be sufficiently accurate in predicting the clay mineral structural and vibrational properties. The classical molecular dynamics (MD) simulations, using the CLAYFF force field, faithfully reproduce the crystal structures with relatively simple analytical functions that include primarily nonbonded interactions. The adsorption properties in these clay minerals are strongly dependent upon the disposition of the hydroxyl group in the octahedral sheet. With the assistance of molecular simulation, the relationship between the hydroxyl group vibrational modes and the molecular-scale structure is explored. The talc hydroxyl groups are oriented perpendicular to the ab plane, while the presence of the dioctahedral vacancies associated with pyrophyllite significantly alters the hydroxyl group structural and vibrational character. Overall, a detailed comparison between the ab initio and the classical MD structural and vibrational properties provides guidance for future refinements to the empirical force field.