Excited states for molecules containing d elements

Beyond the description of relativistic effects, systems containing transition metals are quite demanding with respect to the treatment of electron correlation. This is particularly true when there is a large covalent contribution to the bonding. Correlation is also important in cases where mixing of atomic asymptotes with different d occupations occurs. It poses tremendous demand 

Large-core pseudopotentials are well known to lead to sizable errors [122-125] we thus recommend the use of small-core ECP s that include semicore orbitals in the valence space [120], 

The accuracy of the pseudopotential fit procedure also plays an important role, like the number of reference states used [26,126]. 

Care should also be taken for the quality of the atomic basis set. As in the case of all-electron calculation, an optimal accuracy is reachable by combining high level correlated treatment and extended atomic basis set. 

Schwerdtfeger et al [124] have systematically tested several pseudopotentials for the heaviest element of group I-B, gold atom and its hycWde. The variation between the results obtained from all valence electron small-core Stuttgart energy-adjusted pseudopotentials [122,127] and all electron Douglas-Kroll calculations for AuH is found to be small (Are = 0.001 A, ADe = 0.03 eV, AtOe = 9 cm ). This demonstrates that the pseudopotential approach is a reliable and 

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