Relativistic GVVPT2 Multireference Perturbation Theory Description of the Electronic States of Y2 and Tc2

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 Y2 and Tc2, 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 21Σg+ and 31Σg+ PECs of Y2. These states, together with 11Σg+, are relevant to discussion of controversial results in the literature that suggest dissociation asymptotes that violate the noncrossing rule. The ground state of Y2 was found to be X5Σu (similar to Sc2) with bond length Re = 2.80 Å, binding energy De = 3.12 eV, and harmonic frequency ωe = 287.2 cm–1, whereas the lowest 11Σg+ state of Y2 was found to lie 0.67 eV above the quintet ground state and had spectroscopic constants Re = 3.21 Å, De = 0.91 eV, and ωe = 140.0 cm–1. Calculations performed on Tc2 include study of the previously uncharacterized relatively low-lying 15Σg+ and 19Σg+ states (i.e., 0.70 and 1.84 eV above 11Σg+, respectively). The ground state of Tc2 was found to be X3Σg with Re = 2.13 Å, De = 3.50 eV, and ωe = 336.6 cm–1 (for the most stable isotope, Tc-98) whereas the lowest 1Σg+ state, generally accepted to be the ground state symmetry for isovalent Mn2 and Re2, was found to lie 0.47 eV above the X3Σg state of Tc2. The results broaden the range of demonstrated applicability of the GVVPT2 method.