Reference Energies for Intramolecular Charge-Transfer Excitations.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Quantum Chemistry

Background:

• Previous studies focused on local and Rydberg transitions in organic compounds.
• Accurate computation of vertical transition energies is crucial for understanding molecular electronic properties.
• Intramolecular charge-transfer (CT) transitions are fundamental to many photochemical and photophysical processes.

Purpose of the Study:

• To provide highly accurate vertical transition energies for intramolecular charge-transfer transitions in π-conjugated molecular systems.
• To benchmark various computational methods, including wave function-based approaches, Green's function methods, and time-dependent density-functional theory (TD-DFT), against accurate reference values.
• To identify computationally efficient methods that can accurately predict CT transition energies.

Main Methods:

• A composite protocol involving linear-response coupled-cluster singles, doubles, and triples (LR-CCSDT) with Dunning's double-ζ basis set, corrected by CC3/CCSDT-3 energies using triple-ζ basis sets.
• Further basis set extrapolation up to augmented correlation-consistent polarized valence quadruple-ζ (aug-cc-pVQZ) at the coupled-cluster singles and doubles (CCSD) and CC2 levels.
• Benchmarking against 17 compounds and 30 transitions using methods like CIS(D), SOPPA, RPA(D), EOM-MP2, CC2, CCSD, CCSD(T)(a)*, CCSDR(3), CCSDT-3, CC3, ADC(2), ADC(3), ADC(2.5), Bethe-Salpeter equation (BSE) on top of partially self-consistent evGW (BSE/evGW@HF and BSE/evGW@PBE0), and various TD-DFT functionals.

Main Results:

• Coupled-cluster (CC) methods including triples, such as CCSD(T)(a)*, CCSDR(3), CCSDT-3, and CC3, yielded small average deviations (≤0.10 eV).
• CC3 was identified as the only chemically accurate method among those tested.
• ADC(2.5) showed good performance with a mean absolute error of 0.11 eV. CC2 and BSE/evGW@PBE0 offered satisfying results with favorable computational scaling (O(N⁵) and O(N⁴), respectively).
• In TD-DFT, ωB97X-D, CAM-B3LYP, and M06-2X performed best, with mean absolute errors around 0.15 eV.

Conclusions:

• High-accuracy coupled-cluster methods, especially CC3, are essential for reliable prediction of intramolecular charge-transfer transition energies.
• ADC(2.5), CC2, and BSE/evGW@PBE0 represent good compromises between accuracy and computational cost.
• TD-DFT functionals like ωB97X-D offer a practical and accurate alternative for large-scale studies of CT transitions.