Covalent contributions to bonding in group 12 dimers M2 (Mn = Zn, Cd, Hg)

The spectroscopic constants (Re, De, ωe) of the weakly bound group 12 dimers Zn2, Cd2 and Hg2 have been calculated using relativistic energy-consistent small-core pseudopotentials, large uncontracted (9s8p7d5f5g) valence basis sets and a coupled cluster singles and doubles correlation treatment with a perturbative estimate of the contributions of triples. Spin-orbit contributions have also been considered for Hg2. The theoretical results (Zn2: 3.959 Å, 0.024 eV, 22 cm−1; Cd2: 3.961 Å, 0.036 eV, 20 cm−1; Hg2: 3.769 Å, 0.044 eV, 19 cm−1) are in satisfactory agreement with experimental data (Zn2: 0.034 eV, 26 cm−1; Cd2: 0.039 eV, 23 cm−1; Hg2: 3.63 Å, 0.043 eV, 19 cm−1). Covalent bonding contributions in addition to pure van der Waals interactions are analyzed in terms of atom-based local occupation number operators and associated interatomic charge fluctuations at the complete active space self-consistent field level.