Long-range exchange limit and dispersion in pure silica zeolites

The roles of the large exchange dimensionless gradient limit, s→∞ , with s = | ∇ n( r)|/( 2k_f( r)n( r)) , k_f = ( 3π^2 n( r))^1/3 , and of dispersion interactions computed by Grimme’s scheme in the context of solids are considered. Two families of recently developed generalized gradient approximation exchange functionals in combination with a suitably calibrated dispersion contribution are studied. Furthermore, the effects of changing the correlation functional or including exact exchange in the calculations also are explored. The results indicate that the large exchange dimensionless gradient limit has a small influence and that the most important contribution for a better description of the structure and energetics of porous materials is dispersion. The functional that provides best overall agreement with the experimental stability trend of a large set of pure silica zeolites is an exchange functional (denoted lsRPBE) based on the modified version of PBE, the exchange functional RPBE, corrected to satisfy the large exchange dimensionless limit, combined with PBE correlation and including a calibrated Grimme dispersion contribution. It outperforms any of the functionals that include exact exchange which were tested. Remarkably, the simple local density approximation does almost as well.