Potential-Energy Curves for the Ground and Several Electronic States of NdO and NdS

Potential energy curves (PECs) were calculated for 21 and 18 electronic states of NdO and NdS molecules, respectively. In each case, static electron correlation effects were described by incomplete model space multiconfiguration self-consistent field wave functions based on an active space that included the most important valence orbitals. Dynamic electron correlation was included by the multireference second-order generalized Van Vleck perturbation theory method. Scalar-relativistic contributions were included by the effective core potential approach, using def2-TZVPP basis sets. Spin-dependent relativistic corrections were determined to be small and negligible for the Nd atom and so were not included in the calculations. The 21 and 18 electronic states of NdO and NdS were predicted to be in the excitation energy range of ∼3.2 and ∼2.7 eV, respectively. The ground electronic states of NdO and NdS were determined as 15H (6s4fσ4fϕ4fδ) and 15H (4fϕ4fπ4fπ6s), with spectroscopic constants: bond length Re = 1.780 and 2.325 Å, and harmonic frequency ωe = 891 and 538 cm–1, respectively

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 2¹Σ_g⁺ and 3¹Σ_g⁺ PECs of Y₂. These states, together with 1¹Σ_g⁺, 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 X⁵Σ_u^– (similar to Sc₂) with bond length <i>R</i>_e = 2.80 Å, binding energy <i>D</i>_e = 3.12 eV, and harmonic frequency ω_e = 287.2 cm^–1, whereas the lowest 1¹Σ_g⁺ state of Y₂ was found to lie 0.67 eV above the quintet ground state and had spectroscopic constants <i>R</i>_e = 3.21 Å, <i>D</i>_e = 0.91 eV, and ω_e = 140.0 cm^–1. Calculations performed on Tc₂ include study of the previously uncharacterized relatively low-lying 1⁵Σ_g⁺ and 1⁹Σ_g⁺ states (i.e., 0.70 and 1.84 eV above 1¹Σ_g⁺, respectively). The ground state of Tc₂ was found to be X³Σ_g^– with <i>R</i>_e = 2.13 Å, <i>D</i>_e = 3.50 eV, and ω_e = 336.6 cm^–1 (for the most stable isotope, Tc-98) whereas the lowest ¹Σ_g⁺ state, generally accepted to be the ground state symmetry for isovalent Mn₂ and Re₂, was found to lie 0.47 eV above the X³Σ_g^– state of Tc₂. The results broaden the range of demonstrated applicability of the GVVPT2 method