Low-Lying Electronic States of the Nickel Dimer

The generalized Van Vleck second order multireference perturbation theory (GVVPT2) method was used to investigate the low-lying electronic states of Ni₂. Because the nickel atom has an excitation energy of only 0.025 eV to its first excited state (the least in the first row of transition elements), Ni₂ has a particularly large number of low-lying states. Full potential energy curves (PECs) of more than a dozen low-lying electronic states of Ni₂, resulting from the atomic combinations ³F₄ + ³F₄ and ³D₃ + ³D₃, were computed. In agreement with previous theoretical studies, we found the lowest lying states of Ni₂ to correlate with the ³D₃ + ³D₃ dissociation limit, and the holes in the d-subshells were in the subspace of delta orbitals (i.e., the so-dubbed δδ-states). In particular, the ground state was determined as X ¹Γ_g and had spectroscopic constants: bond length (R _e) = 2.26 Å, harmonic frequency (ω_e) = 276.0 cm^-1, and binding energy (D _e) = 1.75 eV; whereas the 1 ¹Σ_g ⁺ excited state (with spectroscopic constants: R _e = 2.26 Å, ω_e = 276.8 cm^-1, and D _e = 1.75) of the ³D₃ + ³D₃ dissociation channel lay at only 16.4 cm^-1 (0.002 eV) above the ground state at the equilibrium geometry. Inclusion of scalar relativistic effects through the spin-free exact two component (sf-X2C) method reduced the bond lengths of both of these two states to 2.20 Å, and increased their binding energies to 1.95 eV and harmonic frequencies to 296.0 cm^-1 for X ¹Γ_g and 297.0 cm^-1 for 1 ¹Σ_g ⁺. These values are in good agreement with experimental values of R _e = 2.1545 ± 0.0004 Å, ω_e = 280 ± 20 cm^-1, and D = 2.042 ± 0.002 eV for the ground state. All states considered within the ³F₄ + ³F₄ dissociation channel proved to be energetically high-lying and van der Waals-like in nature. In contrast to most previous theoretical studies of Ni₂, full PECs of all considered electronic states of the molecule were produced.