AN ASSESSMENT OF DENSITY-FUNCTIONAL METHODS FOR STUDYING MOLECULAR ADSORPTION IN CLUSTER-MODELS OF ZEOLITES

The performance of six density functional theory (DFT) methods has bee n tested for a zeolite cluster containing three tetrahedral atoms (3T) and the complexes it forms with water and methanol molecules. The DFT methods (BLYP, BP86, BPW91, B3LYP, B3P86, B3PW91) give results in goo d agreement with second-order perturbation theory (MP2). The results i n this paper provide evidence of the suitability of DFT methods for st udying hydrogen-bonded adsorption complexes in zeolites. Generally, th e hybrid DFT methods are in closer agreement with experiment and MP2 t han the pure DFT methods for geometrical parameters. The only exceptio n is the Z(-) geometry, perhaps due to its anionic character. All DFT methods give results in good overall agreement with MP2 for intramolec ular geometrical parameters of the adsorption complexes, intramolecula r vibrational frequencies, and adsorption energies. The B3LYP method g ives intermolecular geometries and intermolecular vibrational frequenc ies which are closest to those obtained from the MP2 method. Thus, the B3LYP method seems to be the best choice for a density functional tre atment of molecular adsorption in zeolite systems. (C) 1998 Elsevier S cience B.V.