Clusters size-consistent state-specificity

Mahapatra, U. S. Datta, B. Mukheijee, D. A size-consistent state-specific multireference coupled cluster theory Formal developments and molecular applications, J. Chem. Phys. 1999,110, 6171-6188. 

A more accurate but computationally expensive set of multistate Gaussian options is provided by the SAC-CI keyword, based on coupled-cluster (CC) techniques for incorporating electron correlation and size-consistency corrections for higher-level excitations. The SAC-CI options cover a wide range of excitation and ionization phenomena, including provision for spin state and symmetry control with analytic derivatives and density corrections for specific target roots. Thus, SAC-CI can offer benchmark-like accuracy for assessing lower-level CI/CAS methods, but involves considerable increase in computational cost and input-keyword complexity. 

Facile dehydrogenation is consistent with kinetic models derived from catalytic conversion studies of cyclohexane to benzene. These models predict an ensemble size for the active site of only one atom. On the other hand, surface science studies propose a model where several metal atoms, on the order of seven, are required, and suggest that specific orientation with respect to subsurface metal atoms is needed. Theoretical studies suggest that the key is to bring cyclohexane sufficiently close to the metal such that strong orbital overlap will occur. Small clusters may be even more effective than surfaces. Further experiments are needed to identify the chemical state of the products of the cluster reactions in order to connect the results with the surface science and catalysis results. 

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