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Authors: Ilka Hegemann, Karin Fink and Volker Staemmler
Conventional wave function based quantum chemical cluster methods can be easily applied for the calculation of adsorption energies, e.g. for van der Waals interactions between (closed shell) molecules and solid surfaces. However, they become exceedingly expensive when the size of the cluster representing the surface is enlarged. This holds true in particular for coupled cluster methods (CEPA. CCSD, CCSD(T)), but to a lesser extent also for MP2. The method of local increments as proposed by Stoll, Paulus and others [1,2] provides a promising alternative procedure. One starts from a Hartree-Fock calculation for the full system (adsorbate + solid surface) and decomposes its correlation energy into contributions of subsystems of increasing size, for instance into one-body, two-body, three-body and higher increments. The individual increments can be obtained by much cheaper calculations in which only part of the total correlation energy is calculated at a time. This allows also to treat the subsystems with extended basis sets which would be prohibitively large for the full system. In the present talk we exploit the possibilities and problems of this incremental scheme for two model adsorption systems. The first one is the interaction of CO with MgO(100) which can be considered as a prototype for a true van der Waals interaction [3]. The second example is the chemisorption of single Cu atoms on defects at different ZnO surfaces. This system is more complicated since it contains an open shell atom and since the adsorption is a genuine chemical interaction with partial charge transfer character. Therefore, the convergence of the incremental scheme has to be monitored with some care. 1. H. Stoll, B. Paulus, P. Fulde, J. Chem. Phys. 123, 144108 (2005) 2. B. Paulus, Physics Reports 428, 1 (2006) 3. V. Staemmler, Top. Organomet. Chem. 12, 219 (2005) |
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