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Alternative single-reference coupled cluster approaches for multireference problems: the simpler, the better.

Francesco A Evangelista1

  • 1Institut für Physikalische Chemie, Universität Mainz, D-55099 Mainz, Germany. evangeli@uni-mainz.de

The Journal of Chemical Physics
|June 21, 2011
PubMed
Summary

New coupled cluster theories offer more accurate calculations for Ne, HF, and C(2) properties. However, the simplest coupled cluster theory remains the most effective overall due to accuracy and cost.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • Coupled cluster (CC) theory is a powerful quantum chemical method for describing electron correlation.
  • Single-reference CC methods are widely used but can struggle with systems requiring multiple electronic configurations.
  • Alternative formulations of CC theory aim to improve accuracy and applicability.

Purpose of the Study:

  • To implement and evaluate alternative single-reference coupled cluster (CC) formulations: extended, unitary, and variational.
  • To assess the accuracy of these methods for molecular properties and potential energy curves.
  • To compare their performance against traditional CC and multireference CC approaches.

Main Methods:

  • General implementation of extended, unitary, and variational CC theories with arbitrary-order truncation.
  • Application to compute energies and equilibrium properties of Ne, HF, and C(2).
  • Calculation of potential energy curves for HF and BeH(2) dissociation, comparing with established multireference CC methods.

Main Results:

  • Alternative CC methods provided highly accurate bond lengths, vibrational frequencies, and anharmonic constants for Ne, HF, and C(2), surpassing traditional CC theory.
  • For potential energy curves, alternative CC methods were more accurate than traditional CC but less accurate than multireference CC.
  • The BeH(2) model revealed a strong dependence of alternative CC theories on the Fermi vacuum choice.

Conclusions:

  • Alternative CC formulations offer improved accuracy for certain molecular properties compared to traditional CC.
  • These methods are not cost-competitive with traditional CC theory when considering accuracy-to-cost ratio.
  • The simplest coupled cluster theory often proves to be the most effective overall.