Multireference Correlated Oscillator Strengths from Adiabatic Connection Approaches Based on Extended Random Phase...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Spectroscopy

Background:

• Accurate calculation of electronic excitation properties, such as oscillator strengths, is crucial for understanding molecular behavior and spectra.
• Traditional methods like CASPT2 and NEVPT2, while accurate, can be computationally demanding, especially for larger systems.
• Adiabatic Connection (AC) methods offer a promising alternative for calculating electronic properties with potentially lower computational cost.

Purpose of the Study:

• To demonstrate the accuracy of adiabatic connection (AC) approaches combined with extended random phase approximation (ERPA) and multireference (CAS) wave functions for calculating oscillator strengths.
• To introduce and evaluate perturbation-corrected ERPA transition density matrices and AC correlation energy methods (AC0, AC0D) for excitation energy and oscillator strength calculations.
• To assess the computational scaling and efficiency of the new AC methods compared to established multiconfigurational perturbation theories.

Main Methods:

• Utilized adiabatic connection (AC) theory combined with extended random phase approximation (ERPA) and complete active space (CAS) wave functions.
• Developed and applied perturbation-corrected ERPA transition density matrices for oscillator strength calculations.
• Employed AC0 and AC0D methods for calculating excitation energies and tested AC0D for triplet excitations.

Main Results:

• Accurate oscillator strengths were obtained using the AC-ERPA-CAS approach.
• The AC0 and AC0D methods provide excitation energies competitive with CASPT2, but with significantly reduced computational effort.
• AC0 and AC0D exhibit favorable scaling with the number of active orbitals compared to CASPT2 and NEVPT2.
• A novel, cost-free method for computing correlated transition dipole moments was developed.
• The AC0D correction improved accuracy for singlet and triplet excitation energies, especially for low-lying states.

Conclusions:

• Adiabatic connection methods, particularly AC0 and AC0D, provide a computationally efficient and accurate route to calculating electronic excitation properties.
• The developed perturbation-corrected ERPA transition density matrices and AC methods offer a viable alternative to traditional high-level computational approaches.
• These findings pave the way for more accessible and accurate theoretical spectroscopy of molecular systems.