Double-shell effect, transition metal

In the tetrahedral Ni(CO)4 complex we have a formal d10 system and there is no CO to Ni a donation. We therefore need no CO a orbitals in the active space. Instead we add empty orbitals of the same symmetry as the 3d orbitals, e and t2. These orbital will turn out to be a mixture of CO tt orbitals and Cr 3d and thus include the double shell effect. The lOinlO active space turns out to be quite general and can be used for many transition metal complexes. This active space will allow studies of the ground state and ligand field excited states. If charge transfer states are considered, one has to extend the active space with the appropriate ligand orbitals. 

State average orbitals are not optimized for a specific electronic state. Normally, this is not a problem and a subsequent CASPT2 calculation will correct for most of it because the first order wave function contains CFs that are singly excited with respect to the CASSCF reference function. However, if the MOs in the different excited states are very different it may be needed to extend the active space such that it can describe the differences. A typical example is the double shell effect that appears for the late first row transition metals as described above. 

One of the most important correlation effects in transition metal systems is the so-called 3d double-shell effect. This correlation effect appears in particular in... 

Introduction Anions have a strong tendency to adsorb specifically at metal surfaces, for example, to estabKsh a direct bond with the electrode by partial loss of their hydration shell. As a consequence of the contact with the electrode, the ionic character of the anions is markedly reduced, resulting in a higher surface concentration than in case of nonspecific adsorption. This effect was first observed in double-layer studies on mercury [229, 230] and later confirmed and studied in detail on single-crystal solid electrodes [231-234]. Specifically adsorbed anions can form various types of ordered structures, either more open (cf. sulfate on Au(hld) [235, 236]) or close-packed as reported for halides on different solid electrodes [21]. Cyclic current-potential curves often reveal sharp current peaks, indicative of phase transitions within the anionic adlayers and hence of the existence of ordered phases [21, 237]. Thermodynamic data of specific anion adsorption was obtained in surface tension studies (on mercury only [229,238-240]), capacitance measurements [231-233], cyclic voltammetry, and chronocoulometry [234]. As an... 

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