Relativistic GVVPT2 Multireference Perturbation Theory Description of the Electronic States of Y<sub>2</sub> and Tc<sub>2</sub>

The multireference generalized Van Vleck second-order perturbation theory (GVVPT2) method is used to describe full potential energy curves (PECs) of low-lying states of second-row transition metal dimers Y<sub>2</sub> and Tc<sub>2</sub>, with scalar relativity included via the spin-free exact two-component (sf-X2C) Hamiltonian. Chemically motivated incomplete model spaces, of the style previously shown to describe complicated first-row transition metal diatoms well, were used and again shown to be effective. The studied states include the previously uncharacterized 2<sup>1</sup>Σ<sub>g</sub><sup>+</sup> and 3<sup>1</sup>Σ<sub>g</sub><sup>+</sup> PECs of Y<sub>2</sub>. These states, together with 1<sup>1</sup>Σ<sub>g</sub><sup>+</sup>, are relevant to discussion of controversial results in the literature that suggest dissociation asymptotes that violate the noncrossing rule. The ground state of Y<sub>2</sub> was found to be X<sup>5</sup>Σ<sub>u</sub><sup>–</sup> (similar to Sc<sub>2</sub>) with bond length <i>R</i><sub>e</sub> = 2.80 Å, binding energy <i>D</i><sub>e</sub> = 3.12 eV, and harmonic frequency ω<sub>e</sub> = 287.2 cm<sup>–1</sup>, whereas the lowest 1<sup>1</sup>Σ<sub>g</sub><sup>+</sup> state of Y<sub>2</sub> was found to lie 0.67 eV above the quintet ground state and had spectroscopic constants <i>R</i><sub>e</sub> = 3.21 Å, <i>D</i><sub>e</sub> = 0.91 eV, and ω<sub>e</sub> = 140.0 cm<sup>–1</sup>. Calculations performed on Tc<sub>2</sub> include study of the previously uncharacterized relatively low-lying 1<sup>5</sup>Σ<sub>g</sub><sup>+</sup> and 1<sup>9</sup>Σ<sub>g</sub><sup>+</sup> states (i.e., 0.70 and 1.84 eV above 1<sup>1</sup>Σ<sub>g</sub><sup>+</sup>, respectively). The ground state of Tc<sub>2</sub> was found to be X<sup>3</sup>Σ<sub>g</sub><sup>–</sup> with <i>R</i><sub>e</sub> = 2.13 Å, <i>D</i><sub>e</sub> = 3.50 eV, and ω<sub>e</sub> = 336.6 cm<sup>–1</sup> (for the most stable isotope, Tc-98) whereas the lowest <sup>1</sup>Σ<sub>g</sub><sup>+</sup> state, generally accepted to be the ground state symmetry for isovalent Mn<sub>2</sub> and Re<sub>2</sub>, was found to lie 0.47 eV above the X<sup>3</sup>Σ<sub>g</sub><sup>–</sup> state of Tc<sub>2</sub>. The results broaden the range of demonstrated applicability of the GVVPT2 method.