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 Y(2) and Tc(2), 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(1)Σ(g)(+) and 3(1)Σ(g)(+) PECs of Y(2). These states, together with 1(1)Σ(g)(+), are relevant to discussion of controversial results in the literature that suggest dissociation asymptotes that violate the noncrossing rule. The ground state of Y(2) was found to be X(5)Σ(u)(–) (similar to Sc(2)) with bond length R(e) = 2.80 Å, binding energy D(e) = 3.12 eV, and harmonic frequency ω(e) = 287.2 cm(–1), whereas the lowest 1(1)(g)(+) state of Y(2) was found to lie 0.67 eV above the quintet ground state and had spectroscopic constants R(e) = 3.21 Å, D(e) = 0.91 eV, and ω(e) = 140.0 cm(–1). Calculations performed on Tc(2) include study of the previously uncharacterized relatively low-lying 1(5)Σ(g)(+) and 1(9)Σ(g)(+) states (i.e., 0.70 and 1.84 eV above 1(1)Σ(g)(+), respectively). The ground state of Tc(2) was found to be X(3)Σ(g)(–) with R(e) = 2.13 Å, D(e) = 3.50 eV, and ω(e) = 336.6 cm(–1) (for the most stable isotope, Tc-98) whereas the lowest (1)Σ(g)(+) state, generally accepted to be the ground state symmetry for isovalent Mn(2) and Re(2), was found to lie 0.47 eV above the X(3)Σ(g)(–) state of Tc(2). The results broaden the range of demonstrated applicability of the GVVPT2 method.