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Jason N Byrd1, Victor F Lotrich1, Rodney J Bartlett1

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A new method, Configuration Interaction Singles-Coupled Cluster Perturbation Theory (CIS-CCPT), improves excitation energy calculations. This approach offers accuracy comparable to Equation of Motion Coupled Cluster (EOM-CC), outperforming CIS(D).

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • Configuration Interaction Singles (CIS) is a common method for calculating excitation energies but lacks accuracy.
  • Existing corrections like CIS(D) improve upon CIS but have limitations.
  • Coupled Cluster methods offer high accuracy but are computationally expensive.

Purpose of the Study:

  • To develop a new state-specific correlation correction for CIS excitation energies.
  • To introduce a method that is size-extensive and incorporates infinite-order effects.
  • To improve the accuracy and consistency of excited state calculations.

Main Methods:

  • Derivation of general expressions for CIS-Coupled Cluster Perturbation Theory (CIS-CCPT).
  • Expansion of CIS-CCPT to first-order in the wavefunction and second-order in the energy.
  • Application and testing of the CIS-CCPT2 method on small organic molecules.

Main Results:

  • CIS-CCPT2 provides a balanced singles space excited state theory.
  • The method demonstrates a standard deviation error of 0.18 eV for excitation energies and 0.14 eV for density of states compared to EOM-CC.
  • CIS-CCPT2 shows a two-fold improvement over CIS(D) with reduced maximum deviations.

Conclusions:

  • CIS-CCPT2 is a significant improvement over CIS(D) for calculating excitation energies.
  • The accuracy of CIS-CCPT2 is comparable to the more computationally demanding EOM-CC method.
  • This new method offers a more accurate and reliable approach for studying excited states in molecules.