ADSORPTION ENERGIES OF NH3 AND NH4-RANGE ELECTROSTATIC POTENTIAL OF THE CRYSTAL( IN ZEOLITES CORRECTED FOR THE LONG)

The calculation of adsorption energies in zeolites in the cluster appr oximation has the advantage of a reduced computational effort compared to that of the periodic approach. However, in the cluster approximati on, the long-range electrostatic effects of the infinite crystal are i gnored and there are boundary effects. In order to remove apart of the disadvantages of the cluster approach, we developed a method to embed a cluster in, a zeolite crystal by imposing an electrostatic potentia l on it. This potential adds the long-range electrostatic effects of t he crystal and subtracts the electrostatic potential of the boundary o f the cluster. It is calculated from the charge distribution of the cr ystal obtained with an ab initio calculation using the CRYSTAL program . We calculated the adsorption energies of NH3 and NH4+ on three diffe rent clusters embedded in the potential of a chabazite crystal, and we have compared them with the adsorption energies of NH3 and NH4+ in th e crystal. If a cluster is used that has dangling bonds four, or more, bonds away from the adsorbate, i.e., the adsorption site is described well covalently, embedding the cluster reduces the error in adsorptio n energies (relative to the crystal calculations) from approximate to 20 to approximate to 2-3 kJ/mol.