Accurate Excited-State Geometries: A CASPT2 and Coupled-Cluster Reference Database for Small Molecules

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate excited-state geometries are crucial for understanding molecular properties and reactions.
• Evaluating the performance of various computational methods is essential for selecting cost-effective approaches.

Purpose of the Study:

• To establish reference values for excited-state structural parameters.
• To assess the accuracy of computationally inexpensive methods for excited-state geometry calculations.

Main Methods:

• Comparison of excited-state geometries calculated using ADC(2), CC2, CCSD, CCSDR(3), CC3, and CASPT2 methods.
• Utilized large atomic basis sets, specifically aug-cc-pVTZ.
• Statistical analysis of deviations between calculated and reference geometries.

Main Results:

• CASPT2 and CC3 methods show excellent agreement for excited-state geometries when all electrons are correlated.
• Excited-state bond lengths are more sensitive to the level of theory than ground-state lengths.
• CCSDR(3) emerges as a cost-effective method for accurate excited-state structures, outperforming CCSD, CC2, and ADC(2).

Conclusions:

• CCSDR(3) provides reliable excited-state geometries and is recommended as a cost-effective method.
• CASPT2 and CC3 are highly accurate but computationally more demanding.
• Reference geometries for medium-sized molecules using CCSDR(3) are provided for future studies.