AUTHOR=Tamukong Patrick K. , Hoffmann Mark R. 

TITLE=Low-Lying Electronic States of the Nickel Dimer

JOURNAL=Frontiers in Chemistry

VOLUME=9

YEAR=2021

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2021.678930

DOI=10.3389/fchem.2021.678930

ISSN=2296-2646

ABSTRACT=<p>The generalized Van Vleck second order multireference perturbation theory (GVVPT2) method was used to investigate the low-lying electronic states of Ni<sub>2</sub>. 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), Ni<sub>2</sub> has a particularly large number of low-lying states. Full potential energy curves (PECs) of more than a dozen low-lying electronic states of Ni<sub>2</sub>, resulting from the atomic combinations <sup>3</sup>F<sub>4</sub> + <sup>3</sup>F<sub>4</sub> and <sup>3</sup>D<sub>3</sub> + <sup>3</sup>D<sub>3</sub>, were computed. In agreement with previous theoretical studies, we found the lowest lying states of Ni<sub>2</sub> to correlate with the <sup>3</sup>D<sub>3</sub> + <sup>3</sup>D<sub>3</sub> 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 <sup>1</sup>Γ<sub>g</sub> and had spectroscopic constants: bond length (<italic>R</italic><sub>e</sub>) = 2.26 Å, harmonic frequency (ω<sub>e</sub>) = 276.0 cm<sup>−1</sup>, and binding energy (<italic>D</italic><sub>e</sub>) = 1.75 eV; whereas the 1 <sup>1</sup>Σ<sub>g</sub><sup>+</sup> excited state (with spectroscopic constants: <italic>R</italic><sub>e</sub> = 2.26 Å, ω<sub>e</sub> = 276.8 cm<sup>−1</sup>, and <italic>D</italic><sub>e</sub> = 1.75) of the <sup>3</sup>D<sub>3</sub> + <sup>3</sup>D<sub>3</sub> dissociation channel lay at only 16.4 cm<sup>−1</sup> (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<sup>−1</sup> for X <sup>1</sup>Γ<sub>g</sub> and 297.0 cm<sup>−1</sup> for 1 <sup>1</sup>Σ<sub>g</sub><sup>+</sup>. These values are in good agreement with experimental values of <italic>R</italic><sub>e</sub> = 2.1545 ± 0.0004 Å, ω<sub>e</sub> = 280 ± 20 cm<sup>−1</sup>, and <italic>D</italic><sub><italic>0</italic></sub> = 2.042 ± 0.002 eV for the ground state. All states considered within the <sup>3</sup>F<sub>4</sub> + <sup>3</sup>F<sub>4</sub> dissociation channel proved to be energetically high-lying and van der Waals-like in nature. In contrast to most previous theoretical studies of Ni<sub>2</sub>, full PECs of all considered electronic states of the molecule were produced.</p>