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Variational coupled cluster for ground and excited states.

Antoine Marie1, Fábris Kossoski1, Pierre-François Loos1

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Summary
This summary is machine-generated.

This study compares variational coupled-cluster (VCC) and traditional CC methods for electronic structure calculations. It explores the energy landscape of paired double excitations (pCCD) and their accuracy against configuration interaction methods.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Electronic Structure Theory

Background:

  • Coupled-cluster (CC) methods are essential for accurate electronic structure calculations, involving solving non-linear equations for amplitudes.
  • While typically used for ground states, CC methods can access higher-energy solutions, potentially representing excited states.
  • Paired double excitations (pCCD) offer a simplified yet relevant model for studying CC method behavior.

Purpose of the Study:

  • To explore the energy landscape of variational coupled-cluster (VCC) methods.
  • To compare the performance of variational pCCD (VpCCD) against traditional pCCD.
  • To evaluate these methods against doubly occupied configuration interaction (CI) using various reference orbitals.

Main Methods:

  • Investigated two model systems: linear and rectangular H4 molecules, subjected to weak and strong correlation regimes.
  • Employed variational pCCD (VpCCD) and traditional pCCD.
  • Used reference Slater determinants from ground-state, excited-state Hartree-Fock, and state-specific optimized orbitals.

Main Results:

  • The study analyzes the energy landscape structure of VpCCD and compares it with traditional pCCD.
  • Performance was gauged against doubly occupied CI, revealing insights into the accuracy of VpCCD with different orbital choices.
  • The influence of spatial symmetry breaking on the VpCCD and pCCD methods was also examined.

Conclusions:

  • Variational coupled-cluster methods offer a pathway to explore complex energy landscapes beyond traditional ground-state solutions.
  • The choice of reference orbitals significantly impacts the accuracy of VpCCD and pCCD, especially under strong correlation.
  • Understanding the energy landscape is crucial for developing robust quantum chemical methods for diverse electronic systems.