A MODEL FOR ELECTRON-ZEOLITE NA- FRAME CHARGES AND IONIC SIZES( ZEOLITE INTERACTIONS )

We construct and test empirical potentials for the interaction of the sodalite framework with an electron injected inside the zeolite and wi th the alkali counterions present in the cage. The electron interacts with Gaussian charge densities centered at the position of each zeolit e atom and characterized by a net charge and a size. The magnitude of the charge is provided by ab initio calculations or by electronegativi ty arguments and the size by the crystal ionic radius. The ion-frame i nteraction contains a short-range Born-Mayer repulsion, as well as a d ispersion and an induced dipole-monopole interaction. In this model th e Coulombic interaction between the counterion and the frame atom uses the same frame charges as the ones used in the electron-frame interac tion, and not the formal charges, commonly used in many ion-frame pote ntials. The effect of the long-range Coulombic interaction between the counterions and the counterions and the frame is calculated by using the Ewald summation. The electron-frame Coulombic interaction is calcu lated by using a fast Fourier transform method to solve Poisson's equa tion. These potentials are tested by calculating the absorption spectr um of an electron ''solvated'' in dry sodalite and in halosodalites. T he computed peak frequencies agree well with those measured. In partic ular, we reproduce the linear dependence of the maximum-absorption wav elength on the lattice constant.