The extended explicitly-correlated second-order approximate coupled-cluster singles and doubles ansatz suitable for...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster methods are essential for accurate electronic structure calculations.
• Approximate coupled-cluster methods offer a balance between accuracy and computational cost.
• Explicit correlation techniques improve convergence to the basis-set limit.

Purpose of the Study:

• To develop an extended explicitly correlated approximate coupled-cluster singles and doubles (CC2) method for response properties.
• To implement the CC2(F12*)-XSP method in the KOALA program using automated equation derivation and density fitting.
• To assess the accuracy and efficiency of the CC2(F12*)-XSP method for predicting excitation energies.

Main Methods:

• Development of the CC2(F12*)-XSP method incorporating explicit correlation.
• Automated derivation and hand-coding of equations into the KOALA program.
• Application of density fitting for all two-electron integrals.
• Calculation of vertical singlet excitation energies for selected molecules.

Main Results:

• The CC2(F12*)-XSP method achieves the correct basis-set limit without ground-state bias.
• Excellent agreement was found between CC2(F12*)-XSP and reference CC2 values using large basis sets.
• Excitation energies calculated with the aug-cc-pVTZ basis were converged within 1 mEh to the basis-set limit for valence excitations.

Conclusions:

• The CC2(F12*)-XSP method is a reliable and efficient approach for calculating response properties, particularly excitation energies.
• The method provides accurate results comparable to high-level coupled-cluster calculations.
• This development offers a computationally feasible route to accurate electronic excitation energies.