The bond nature in beryllium dimer
has been theoretically investigated
using high-level ab initio methods. A series of ANO
basis sets of increasing quality, going from sp to spdf ghi contractions,
has been employed, combined with HF, CAS-SCF, CISD, and MRCI calculations
with several different active spaces. The quality of these calculations
has been checked by comparing the results with valence Full-CI calculations,
performed with the same basis sets. It is shown that two quasi-degenerated
partly occupied orbitals play a crucial role to give a qualitatively
correct description of the bond. Their nature is similar to that of
the edge orbitals that give rise to the quasi-degenerated singlet–triplet
states in longer beryllium chains.
The electronic structure and some electron transfer properties of a model mixed-valence Spiro molecular cation have been investigated at CAS-SCF, CAS+S, and CAS+SD levels starting from canonical and localized orbitals, using SZ, DZ, and TZP basis sets. The potential energy surfaces of the adiabatic ground and the lowest three excited electronic states have been computed, within a two-state model, and a double-well potential has been obtained for the ground electronic state. We have demonstrated the low coupling interaction between the two redox moieties of this molecular cation by following the charge localization/delocalization in the valence pi system through the reaction coordinate of the intramolecular charge transfer. The effect of dynamical correlation, using either localized or canonical orbitals, was found to be crucial for a quantitative description of the electronic structure and some important electron transfer parameters of this mixed-valence system.
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