Understanding the chemical bonding of ground and excited states of HfO and HfB with correlated wavefunction theory and density functional approximations

Knowledge of the chemical bonding of HfO and HfB ground and low-lying electronic states provides essential insights into a range of catalysts and materials that contain Hf-O or Hf-B moieties. Here, we carry out high level multi-reference configuration interaction theory and coupled cluster quantum calculations on these systems. We compute full potential energy curves, excitation energies, ionization energies, electronic configurations, and spectroscopic parameters with large quadruple- ζ and quintuple- ζ quality correlation consistent basis sets. We also investigate equilibrium chemical bonding patterns and effect of correlating core electrons on property predictions. Differences in the ground state electron configuration of HfB (X 4 Ʃ − ) and HfO (X 1 Ʃ + ) leads to a significantly stronger bond in HfO than HfB, as judged by both dissociation energies and equilibrium bond distances. We extend our analysis to the chemical bonding patterns of the isovalent HfX (X = O, S, Se, Te, Po) series and observe similar trends. We also note a linear trend between the decreasing value of D e from HfO to HfPo and the singlet-triplet energy gap (ΔE S-T ) of the molecule. Finally, we compare these benchmark results to those obtained using density functional theory (DFT) with 23 exchange-correlation functionals spanning multiple rungs of the "Jacob's ladder". When comparing DFT errors to coupled cluster reference values on dissociation energies, excitation energies, and ionization energies of HfB and HfO, we observe semi-local GGAs to significantly outperform more complex and high-cost functionals.