A modified ansatz for explicitly correlated coupled-cluster wave functions that is suitable for response theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster (CC) wave functions are essential for accurate electronic structure calculations.
• Explicitly correlated methods incorporate inter-electron distances to improve accuracy.
• Existing methods may struggle with excited states and specific electronic configurations.

Purpose of the Study:

• To develop a modified ansatz for explicitly correlated coupled-cluster wave functions.
• To enable accurate description of singly excited configurations within the correlated framework.
• To improve the computational treatment of molecular electronic states.

Main Methods:

• A modified ansatz based on the fixed amplitude approach is proposed.
• An additional term is incorporated for explicitly correlated singly excited configurations.
• The method is implemented for coupled-cluster singles and doubles (CCSD) using automated techniques.

Main Results:

• Numerical results for vertical excitation energies, equilibrium distances, and harmonic frequencies of diatomics are presented.
• The new approach provides an unbiased description of ground and singly excited states.
• Improvements over conventional orbital expansions are observed for both ground and excited states.

Conclusions:

• The modified ansatz effectively enhances explicitly correlated coupled-cluster calculations.
• The approach shows significant promise for accurately describing excited electronic states.
• A correction for one-electron terms is suggested for further refinement, particularly for response theory.