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High-level ab initio CCSD(T) and spin-orbit icMRCI+Q calculations were used to predict potential energy curves (PECs) for the lowest-lying states of ZrO, ZrS, HfO, and HfS. The prediction of the ground state is basis set dependent at the icMRCI+Q level for ZrO and ZrS due to the small singlet-triplet splitting between the lowest 1Σ+ and 3Δ states. CCSD(T) with a spin orbit correction predicted the 1Σ+ ground state in agreement with experiment. New all-electron basis sets were developed for Hf to improve the results over those predicted by use of effective core potentials (ECPs) that subsume the 4f electrons into the definition of the core. The use of the new DK-4f basis sets rather than ECPs became more important for HfO and HfS where there is a lack of a good core-valence separation. icMRCI+Q, CCSD(T), and DFT calculations for the spectroscopic parameters of ZrO, ZrS, HfO, and HfS were benchmarked with available experimental data. Bond dissociation energies (BDEs) of these four systems were calculated at the Feller-Peterson-Dixon (FPD) level to be 762.1 (ZrO), 543.5 (ZrS), 803.8 (HfO), and 575.1 kJ/mol (HfS), in excellent agreement with experiment. The HfS BDE was remeasured using the R3PI method, providing an updated experimental measurement of D0(HfS) = 5.978 ± 0.002 eV = 576.8 ± 0.2 kJ/mol. This experimental value, combined with experimental measurements of the ionization energies of Hf and HfS, gives the cationic BDE of D0(Hf+-S) = 5.124 ± 0.002 eV = 494.4 ± 0.2 kJ/mol. 

The spectroscopic identification of Bi 4 has been very elusive. Two constitutional Bi 4 isomers of T d and C 2v symmetry are investigated and each is found to be a local energetic minimum. The optimized geometries and vibrational frequencies of these two isomers are obtained at the CCSD(T)/cc-pVQZ-PP level of theory, utilizing the Stoll, Metz, and Dolg 60-electron effective core potential. The fundamental frequencies of the T d isomer are obtained at the same level of theory. The focal point analysis method, from a maximum basis set of cc-pV5Z-PP, and proceeding to a maximum correlation method of CCSDTQ, was employed to determine the dissociation energy of Bi 4 ( T d ) into two Bi 2 and the adiabatic energy difference between the C 2v and T d isomers of Bi 4 . These quantities are predicted to be +65 kcal mol −1 and +39 kcal mol −1 , respectively. Two electron vertical excitation energies between the T d and C 2v electronic configurations are computed to be 156 kcal mol −1 for the T d isomer and 9 kcal mol −1 for the C 2v isomer. The most probable approach to laboratory spectroscopic identification of Bi 4 is via an infrared spectrum. The predicted fundamentals (cm −1 ) with harmonic IR intensities in parentheses (km mol −1 ) are 94(0), 123(0.23), and 167(0) for the T d isomer. The moderate IR intensity for the only allowed fundamental may explain why Bi 4 has yet to be observed. Through natural bond orbital analysis, the C 2v isomer of Bi 4 was discovered to exhibit “long-bonding” between the furthest apart ‘wing’ atoms. This long-bonding is postulated to be facilitated by the σ-bonding orbital between the ‘spine’ atoms of the C 2v isomer.