Performance of Equation of Motion-Coupled Cluster Methods for Computing Anharmonic Vibrational Frequencies of...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Accurate prediction of molecular vibrational frequencies and rotational constants is crucial for understanding chemical structures and dynamics.
• Equation of Motion (EOM)-based Coupled Cluster (CC) methods offer a robust framework for calculating these properties.
• Hybrid quartic force fields (QFFs) are employed to incorporate anharmonicity, improving accuracy beyond harmonic approximations.

Purpose of the Study:

• To evaluate the accuracy of EOM-based CC methods, particularly EOM-ionization potential (IP) and EOM-electron affinity (EA), in predicting anharmonic vibrational frequencies.
• To compare the performance of approximate triples (CCSD(T)(a)) and full triples (CCSDT) CC methods for ground and excited electronic states.
• To apply the most accurate methods to characterize the rovibronic states of SiOH and re-evaluate spectroscopic assignments for HCF+.

Main Methods:

• Utilized Equation of Motion (EOM)-based Coupled Cluster (CC) methods, including EOM-IP-CCSD(T)(a) and EOM-IP-CCSDT.
• Employed hybrid quartic force fields (QFFs) to compute anharmonic vibrational frequencies and rotational constants.
• Benchmarked computational results against experimental data for various open-shell molecules (HOO, HNF, HSO, HCF+).

Main Results:

• EOM-based CC methods with hybrid QFFs achieved average errors of 1.5% for ground states and 4.4% for excited states.
• EOM-IP-CCSD(T)(a)/CcCR+TZ QFF demonstrated excellent performance for ground states with a mean absolute percent error of 1.3%.
• Accurate prediction of excited state frequencies necessitates full triples (CCSDT), which significantly increases computational cost (∼70x).

Conclusions:

• EOM-based CC methods provide highly accurate anharmonic vibrational frequencies and rotational constants, essential for molecular characterization.
• The choice between approximate and full triples methods depends on the required accuracy for ground versus excited electronic states.
• The study provides a full rovibronic characterization for SiOH and questions a previous spectroscopic assignment for HCF+.