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Multiple Solutions to the Single-Reference CCSD Equations for NiH.

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Summary
This summary is machine-generated.

Multiple coupled cluster (CC) solutions can arise from a single unrestricted Hartree-Fock (UHF) reference wave function for challenging molecules like NiH. This study investigates these solutions to accurately describe electronic states, suggesting Brueckner orbitals for improved calculations.

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Area of Science:

  • Quantum chemistry
  • Computational chemistry
  • Electronic structure theory

Background:

  • Hartree-Fock (HF) reference wave functions are typically assumed to uniquely determine correlated wave functions for a given electronic state.
  • Unrestricted Hartree-Fock (UHF) calculations can yield multiple solutions, especially for open-shell systems.
  • Coupled Cluster (CC) methods, like CCSD, are powerful tools for describing electron correlation but rely on a suitable reference state.

Purpose of the Study:

  • To investigate the appearance of multiple CCSD solutions originating from a UHF reference wave function for the NiH molecule.
  • To establish a reliable method for correlating these multiple CCSD solutions with distinct physical electronic states.
  • To identify potential strategies for improving the robustness of HF-based coupled cluster calculations in challenging cases.

Main Methods:

  • Utilized unrestricted Hartree-Fock (UHF) calculations to obtain reference wave functions for NiH.
  • Performed Coupled Cluster Singles and Doubles (CCSD) calculations using the UHF reference.
  • Analyzed potential energy curves, spin density isovalue plots, and Equation-of-Motion CCSD (EOM-CCSD) calculations to characterize the electronic states corresponding to different CCSD solutions.

Main Results:

  • Observed multiple distinct CCSD solutions when starting from a UHF reference for the ground state of NiH.
  • Demonstrated that these multiple solutions correspond to different electronic states, not just variations of the same state.
  • Identified specific computational diagnostics (potential energy curves, spin densities, EOM-CCSD) to differentiate between these solutions.

Conclusions:

  • The assumption of a unique correlated wave function from a single HF reference is not always valid, particularly with UHF for challenging systems.
  • Careful analysis of CCSD wave function properties is crucial for correctly assigning them to physical electronic states.
  • Employing Brueckner orbitals is recommended to mitigate issues associated with multiple solutions in HF-based coupled cluster calculations for such systems.