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

We construct and test empirical potentials for the interaction of the sodalite framework with an electron injected inside the zeolite and with the alkali counterions present in the cage. The electron interacts with Gaussian charge densities centered at the position of each zeolite atom and characterized by a net charge and a size. The magnitude of the charge is provided by ab initio calculations or by electronegativity arguments and the size by the crystal ionic radius. The ion-frame interaction contains a short-range Born-Mayer repulsion, as well as a dispersion and an induced dipole-monopole interaction. In this model the Coulombic interaction between the counterion and the frame atom uses the same frame charges as the ones used in the electron-frame interaction, and not the formal charges, commonly used in many ion-frame potentials. 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 calculated by using a fast Fourier transform method to solve Poisson's equation. These potentials are tested by calculating the absorption spectrum of an electron ''solvated'' in dry sodalite and in halosodalites. The computed peak frequencies agree well with those measured. In particular, we reproduce the linear dependence of the maximum-absorption wavelength on the lattice constant.