MOLECULAR MECHANICS POTENTIAL FOR SILICA AND ZEOLITE CATALYSTS BASED ON AB-INITIO CALCULATIONS .1. DENSE AND MICROPOROUS SILICA

A consistent force field for the simulation of aluminum-free zeolite structures is presented. The parameters are derived from results of ab initio calculations on molecular models which represent typical structural elements of zeolites: SiO4 tetrahedra connected to chains (disilicic and trisilicic acid), rings ([SiO(OH)(2)](n), n = 3-6), and cages ([SiO3/2(OH)](n), n = 8, 12, 24). These calculations used a ''double zeta + polarization/triple zeta + polarization'' basis set. The structures predicted by means of the force field obtained are compared with the results of direct ab initio calculations of the model molecules and with observed structures of dense and microporous silica. The conclusion is reached that it is possible to derive an accurate and transferable force field for molecules and solids solely based on ab initio data for molecules.