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Analytic Gradients for Selected Configuration Interaction.

Jeremy P Coe1

  • 1Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom.

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|January 19, 2023
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Summary
This summary is machine-generated.

We developed analytic gradients for selected configuration interaction (SCI) wave functions, offering a computationally efficient alternative to full configuration interaction. This method provides accurate results using significantly fewer determinants.

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Area of Science:

  • Quantum chemistry
  • Computational chemistry
  • Electronic structure theory

Background:

  • Selected configuration interaction (SCI) methods offer a balance between accuracy and computational cost in electronic structure calculations.
  • Analytic gradients are crucial for geometry optimization and molecular dynamics simulations.
  • Calculating gradients for SCI wave functions presents unique computational challenges, particularly concerning orbital degeneracies.

Purpose of the Study:

  • To develop and validate analytic gradients for selected configuration interaction (SCI) wave functions.
  • To address computational challenges associated with orbital degeneracies in gradient calculations.
  • To demonstrate the efficiency and accuracy of the developed analytic gradients compared to full configuration interaction (FCI).

Main Methods:

  • Development of analytic gradient expressions for SCI wave functions.
  • Application of coupled perturbed Hartree-Fock (CPHF) equations, with specific handling of degenerate orbital pairs.
  • Introduction and utilization of seminumerical gradients for validation.
  • Testing the method on various molecular systems including CO, NH3, H4, H6, and CH4.

Main Results:

  • Analytic gradients for SCI were successfully developed and implemented.
  • Degenerate orbital pairs from different irreducible representations were shown to be safely excluded from CPHF equations, avoiding instabilities.
  • Seminumerical gradients confirmed the analytic approach's validity, even with near-degeneracies in distorted high-symmetry geometries.
  • The SCI analytic gradients achieved accuracy comparable to FCI while using a fraction of the determinants.

Conclusions:

  • Analytic gradients for SCI provide a computationally feasible and accurate method for electronic structure calculations.
  • The developed method effectively handles orbital degeneracies, enhancing stability and reliability.
  • SCI gradients offer a significant advantage in computational efficiency over FCI for achieving high-quality molecular geometries and properties.