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Updated: Oct 25, 2025

Author Spotlight: Development of a Novel Finite Element Analysis Model for Improved Orthognathic Surgical Techniques
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Complete-active-space extended Koopmans theorem method.

Ernest R Davidson1, Joseph Vincent Ortiz2, Viktor N Staroverov3

  • 1Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, USA.

The Journal of Chemical Physics
|August 8, 2021
PubMed
Summary
This summary is machine-generated.

The complete-active-space (CAS) extended Koopmans theorem (EKT) offers a computationally efficient method for calculating ionization energies. While not always exact, CAS EKT can approximate full configuration interaction (CI) results with high precision.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • The Koopmans' theorem provides a simple approximation for ionization energies.
  • Extended Koopmans' theorem (EKT) methods offer improved accuracy over the original theorem.
  • Complete-active-space (CAS) methods are crucial for accurately describing electron correlation in quantum chemistry.

Purpose of the Study:

  • To define and analyze the complete-active-space (CAS) extended Koopmans theorem (EKT) as a specific variant of EKT.
  • To establish the relationship between CAS EKT and configuration interaction (CI) methods.
  • To clarify the accuracy and limitations of CAS EKT for predicting first ionization energies.

Main Methods:

  • Defining CAS EKT as a restricted form of EKT using a CAS configuration interaction (CI) reference state.
  • Analyzing the electron removal operator's action restricted to active orbitals.
  • Comparing CAS EKT predictions with full CI calculations for ionization energies.

Main Results:

  • Demonstrated the equivalence between CAS EKT and a specific set of CI calculations involving hole-state configurations within the active space.
  • Showed that CAS EKT ionization energies are generally not exact but can approach full CI accuracy.
  • Established that CAS EKT can achieve arbitrary precision within finite basis sets.

Conclusions:

  • CAS EKT is a valuable theoretical tool with properties analogous to the original Koopmans theorem.
  • The method provides a transparent understanding of the accuracy of EKT for ionization energies.
  • The findings reconcile disparate statements regarding EKT in existing scientific literature.