PERIODIC ABINITIO HARTREE-FOCK CALCULATIONS OF THE LOW-SYMMETRY MINERAL KAOLINITE

Periodic ab initio Hartree-Fock LCAO calculations have been carried out on the P1 symmetry clay mineral kaolinite, Al2Si2O9H4 using STO-3G and modified 6-21G basis sets. The three structural degrees of freedom associated with the inner hydrogen atom of this compound have been optimized using the STO-3G basis. The equilibrium position of the inner hydrogen (described relative to the adjoining O-H bond) is predicted to be approximately parallel to the (ab) plane (forming an angle of +3.1-degrees) with an O-H bond length of 0.99 angstrom and the two Al-O-H angles of 107.7-degrees and 107.4-degrees, respectively. The physical properties of the system were obtained using the 6-21G basis at the geometry deduced from the STO-3G calculations. The total valence density of states has been computed. Projected densities of states have been evaluated for the chemically distinct sets of elements. It is found that the majority of the valence states are composed of oxygen 2s and 2p atomic orbitals with overlap from Si and Al 3s and 3p atomic orbitals. Crystal charge densities and density deformation maps have been computed in the basal plane of the silicate ring system. The calculations have revealed small charge densities in the center of the ditrigonal cavity (10(-4)-10(-5) e/bohr3) and an oblate distortion of the oxygen charge density directed toward the center of the ring system. The formal charges resulting from a Mulliken population analysis of the 6-21G data give the following: Si = +2.5 \e\, O = -1.7 \e\, Al = +2.1 \e\, and H = +0.56 \e\. Electrostatic potential maps have been computed in the chemically accessible regions of the structure near the ditrigonal cavities. These maps indicate that the center of the cavity is at a negative potential relative to nearby silicon positions. Analogous calculations have been carried out perpendicular to the layers of the material. The results of these calculations reveal extensive interlayer hydrogen bonding.