Finding transition structures in extended systems: A strategy based on a combined quantum mechanics-empirical valence bond approach

A method for efficient localization and description of stationary points on the potential energy surface of extended systems is presented. It is based on Warshel's empirical valence bond approach, for which we propose a modif ication, and combines the potential function description of the total syste m with a quantum mechanical description of the reaction site (QM-Pot). We d escribe the implementation of the method in the QMPOT program, which is bas ically an optimizer for minima and saddle points and has interfaces to exis ting quantum mechanical (e.g., TURBOMOLE, GAUSSIAN94) and interatomic poten tial function codes (e.g., GULP, DISCOVER). The power of the method is demo nstrated for proton transfer reactions in zeolite catalysts, which may have as many as 289 atoms in the unit cell. As a test case the zeolite chabazit e is considered in this study. Its limited unit cell size (37 atoms) makes comparison with the full periodic ab initio limit possible. The inclusion o f long-range effects due to the periodic crystal structure by the QM-Pot me thod proves crucial in obtaining reliable results. The combined quantum mec hanics-interatomic potential function calculations yield reaction barriers within 6 kJ/mol and reaction energies within 3.5 kJ/mol of the periodic ab initio limit. The zero-point vibrational energy corrected reaction barriers are between 58 and 97 kJ/mol for the six different proton jump paths. Thes e are density functional results employing the B3LYP functional. (C) 2000 A merican Institute of Physics. [S0021-9606(00)30416-0].