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Summary
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Multicomponent quantum chemistry methods can accurately model protonic densities by including electron-proton correlation. This study introduces a new method, multicomponent heat-bath configuration interaction (HCI), revealing inherent multireference character in these systems.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Multicomponent quantum chemical methods aim to incorporate nuclear quantum effects by relaxing the Born-Oppenheimer approximation for specific nuclei.
  • Accurate protonic densities in multicomponent methods necessitate the inclusion of electron-proton correlation.
  • Existing multicomponent methods often rely on single-reference mean-field wave functions, neglecting crucial electron-proton correlation.

Purpose of the Study:

  • To investigate the impact of using single-reference mean-field (Hartree-Fock) orbitals in multicomponent quantum chemical calculations.
  • To develop and apply a novel multicomponent heat-bath configuration interaction (HCI) method for accurate electronic structure calculations.
  • To analyze the multireference character of systems within a multicomponent framework.

Main Methods:

  • Development of the multicomponent heat-bath configuration interaction (HCI) method, a selected configuration interaction technique.
  • Application of the multicomponent HCI method to HeHHe+, HCN, and FHF- systems.
  • Comparison of results with reference grid-based calculations to validate protonic density accuracy.

Main Results:

  • The multicomponent HCI method successfully reproduced the ground-state protonic densities for HeHHe+, HCN, and FHF-.
  • All studied systems exhibited significant multireference character (leading configuration coefficient < 0.95) in the multicomponent framework.
  • This multireference character persisted even when using natural orbitals instead of Hartree-Fock orbitals for FHF- and HCN.

Conclusions:

  • Using Hartree-Fock orbitals in multicomponent calculations can be problematic due to the inherent multireference character of these systems.
  • Multireference character appears to be more common or inherent in multicomponent quantum chemical frameworks compared to single-component ones.
  • The developed multicomponent HCI method provides an accurate approximation for systems with large active spaces and highlights the importance of electron-proton correlation.