Mixed valence character of anionic linear beryllium chains: a CAS-SCF and MR-CI study

A theoretical investigation on the mixed valence behavior, or bistability, of a series of anionic linear chains composed of beryllium atoms is presented. Calculations on Be(N)- (with N = 7, ..., 13) were performed at CAS-SCF and MR-CI levels by using an ANO basis set containing 6s4p3d2f contracted orbitals for each atom. Our results show a consistent gradual shift between different classes of mixed valence compounds as the number of beryllium atoms increases, from strong coupling (class III) toward valence-trapped (class II). Indeed, in the largest cases (N > 10), the anionic chains were found to become asymptotically closer to class I, where the coupling vanishes. The intramolecular electron-transfer parameters V(ab), E(barr), and E(opt) were calculated for each atomic chain. It is shown that the decrease of V(ab) with increasing N follows an exponential pattern.