## Relativistic
GVVPT2 Multireference Perturbation Theory Description of the Electronic
States of Y_{2} and Tc_{2}

journal contribution

posted on 27.02.2014, 00:00 by Patrick K. Tamukong, Mark R. Hoffmann, Zhendong Li, Wenjian LiuThe 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}Σ_{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.