Role of dispersive interactions in layered materials: a periodic B3LYP and B3LYP-D* study of Mg(OH)2, Ca(OH)2 and kaolinite

Abstract

The role of dispersive interactions on the structure, energetic and vibrational features of brucite [Mg(OH)₂], portlandite [Ca(OH)₂] and kaolinite [Al₂Si₂O₅(OH)₄] layered materials has been addressed for the first time. Dispersion contribution is included with a −C₆/R⁶ empirical correction to the B3LYP functional (B3LYP-D* recipe) which has recently been employed to study molecular crystals. To decrease the spurious effect of the basis set superposition error, Gaussian basis sets of triple-zeta plus polarization functions were adopted. Comparing B3LYP and B3LYP-D* results shows the latter to provide significant improvement as far as structure and energetic data are concerned. For the treated systems the cell parameter controlling the inter-layer distance, usually overestimated by the pure B3LYP, is in good agreement with experiment. The inter-layer interaction energy is dramatically increased by the dispersion contribution which, for kaolinite, fully justifies the request of strong Lewis basic molecules needed to swell the material. In general, B3LYP harmonic frequencies are scarcely changed by the dispersive correction albeit some modes sensitive to the inter-layer separation may be significantly perturbed at B3LYP-D*. The present results are encouraging although fine tuning of the proposed empirical correction for inorganic systems might be needed.