A Mountaineering Strategy to Excited States: Revising Reference Values with EOM-CC4

Area of Science:

• Computational chemistry
• Quantum chemistry
• Theoretical chemistry

Background:

• Accurate calculation of vertical excitation energies is crucial for understanding electronic transitions in organic molecules.
• Coupled-cluster (CC) methods provide high accuracy but can be computationally expensive.

Purpose of the Study:

• To evaluate the performance of the approximate fourth-order coupled-cluster (CC4) method for calculating vertical excitation energies.
• To assess the effectiveness of basis set correction schemes in conjunction with CC methods.
• To improve theoretical estimates for the QUEST database.

Main Methods:

• Utilized the equation-of-motion formalism with the CC4 method.
• Compared CC4 results against reference values from coupled-cluster calculations including up to quadruples excitations (CCSDTQ).
• Assessed additive basis set correction schemes involving lower-order CC calculations in larger basis sets.

Main Results:

• CC4 demonstrated excellent agreement with CCSDTQ for excited states dominated by single excitations, with an average deviation of 0.003 eV.
• Basis set correction schemes were found to be beneficial but varied in accuracy depending on the specific scheme.
• CC4 was used to refine theoretical best estimates in the QUEST database for mid-sized organic molecules.

Conclusions:

• The CC4 method is a highly accurate and efficient approximation for calculating vertical excitation energies in small organic compounds.
• Basis set extrapolation techniques can further enhance the accuracy of excitation energy calculations.
• The improved QUEST database estimates provide valuable benchmarks for future computational and experimental studies.