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  1. Bond dissociation energies (BDE) are key descriptors for molecules and are among the most sought-after properties in chemistry. Despite their importance, the accurate prediction of BDE’s for transition metal species can be particularly daunting for both experiment and computation. Experimental data has been limited and, when available, often has large error bars, making the critical evaluation and identification of suitable computational methods difficult. However, recent advancements in the experimental determination of BDE’s with techniques such as Velocity Map Imaging and 2 Photon Ionization now provide useful gauges for computational strategies and new methodologies, providing energies with unprecedented accuracies. The vanadium diatomics (VX, X=B, C, N, O, F, Al, Si, P, S, Cl) have been challenging for computational chemistry methods, and, thus, a new experimental gauge enables methods to be reevaluated and developed for these species. Herein, the super-correlation consistent Composite (super-ccCA or s-ccCA), a new thermochemical scheme centered around CCSD(T)/complete basis set (CBS) limit computations with additional contributions that account for scalar-relativistic effects, and coupled cluster contributions beyond CCSD(T) up to quintuple excitations has been considered. The agreement between determinations made by the s-ccCA scheme and by recent experiment is excellent, demonstrating the utility of the new approach in addressing challenging metal systems, even those of multireference nature. In light of recent experimental BDE’s, the longstanding correlation consistent composite approach (ccCA) is also evaluated for the VX species and find that the mean absolute deviation (MAD) is greatly reduced compared to previously used experimental values. 
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    Free, publicly-accessible full text available December 17, 2024
  2. Free, publicly-accessible full text available August 11, 2024
  3. Abstract

    Vibrational spectroscopy enables critical insight into the structural and dynamic properties of molecules. Presently, the majority of theoretical approaches to spectroscopy employ wavefunction‐basedab initioor density functional methods that rely on the harmonic approximation. This approximation breaks down for large molecules with strongly anharmonic bonds or for molecules with large internuclear separations. An alternative to these methods involves generating molecular anharmonic potential energy surfaces (potentials) and using them to extrapolate the vibrational frequencies. This study examines the efficacy of density functional theory (DFT) and the correlation consistent Composite Approach (ccCA) in generating anharmonic frequencies from potentials of small main group molecules. Vibrational self‐consistent field Theory (VSCF) and post‐VSCF methods were used to calculate the fundamental frequencies of these molecules from their potentials. Functional choice, basis set selection, and mode‐coupling are also examined as factors in influencing accuracy. The absolute deviations for the calculated frequencies using potentials at the ccCA level of theory were lower than the potentials at the DFT level. With DFT resulting in bending modes that are better described than those of ccCA, a multilevel DFT:ccCA approach where DFT potentials are used for single vibrational mode potentials and ccCA is used for vibrational mode‐mode couplings can be utilized for larger polyatomic systems. The frequencies obtained with this multilevel approach using VCIPSI‐PT2 were closer to experimental frequencies than the scaled harmonic frequencies, indicating the success of utilizing post‐VSCF methods to generate more accurate representations of computed infrared spectra.

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  4. The f-block ab initio correlation consistent composite approach was used to predict the dissociation energies of lanthanide sulfides and selenides. Geometry optimizations were carried out using density functional theory and coupled cluster singles, doubles, and perturbative triples with one- and two-component Hamiltonians. For the two-component calculations, relativistic effects were accounted for by utilizing a third-order Douglas–Kroll–Hess Hamiltonian. Spin–orbit coupling was addressed with the Breit–Pauli Hamiltonian within a multireference configuration interaction approach. The state averaged complete active space self-consistent field wavefunctions obtained for the spin–orbit coupling energies were used to assign the ground states of diatomics, and several diagnostics were used to ascertain the multireference character of the molecules. 
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