Correlation correction to configuration interaction singles from coupled cluster perturbation theory

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.