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Comparison of Methods for Active Orbital Selection in Multiconfigurational Calculations.

Zsuzsanna Tóth1,2, Peter Pulay1

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The unrestricted Hartree-Fock natural orbital (UNO) criterion effectively identifies active spaces for complex molecules, matching advanced methods. A new analytical technique resolves previous limitations, making UNO a robust choice for quantum chemistry calculations.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Molecular Modeling

Background:

  • Constructing accurate active orbital spaces is crucial for multiconfigurational wave function calculations.
  • Strong electron correlation is prevalent in various molecular systems, including conjugated systems, transition states, and transition metal complexes.
  • Existing methods for active space selection have limitations in accuracy, computational cost, or applicability.

Purpose of the Study:

  • To compare different methods for constructing active orbital spaces in multiconfigurational calculations.
  • To evaluate the performance of the unrestricted Hartree-Fock natural orbital (UNO) criterion for selecting active spaces.
  • To address challenges associated with the UNO criterion, particularly in obtaining broken spin symmetry solutions.

Main Methods:

  • Comparison of active space selection methods on moderately to strongly correlated ground-state molecules.
  • Application of the unrestricted Hartree-Fock natural orbital (UNO) criterion.
  • Development and application of an analytical method to overcome difficulties in finding broken spin symmetry unrestricted Hartree-Fock solutions.

Main Results:

  • The UNO criterion successfully identifies active spaces comparable to more computationally expensive approximate full configuration interaction (CI) methods for the studied molecules.
  • A novel analytical method accurately determines broken spin symmetry unrestricted Hartree-Fock solutions, resolving a key disadvantage of the UNO criterion.
  • The UNO criterion considers both energetic proximity to the Fermi level and exchange interactions, providing a measure of correlation strength for orbital selection.

Conclusions:

  • The UNO criterion is a simple, effective, and computationally advantageous method for selecting active spaces in multiconfigurational calculations.
  • The developed analytical method enhances the reliability and applicability of the UNO criterion.
  • UNO criterion offers a robust approach for estimating correlation strength, valuable for Restricted Active Space methods.