Assignment of vibrational states within configuration interaction calculations

Area of Science:

• Quantum Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Vibrational state assignment is crucial for analyzing experimental infrared spectra using quantum chemical calculations.
• Standard methods using configuration interaction vectors may fail with localized coordinates or non-Abelian molecules.
• Complexities arise when combining special coordinate systems and real-valued configuration bases.

Purpose of the Study:

• To develop an efficient and general method for assigning vibrational states in quantum chemical calculations.
• To overcome limitations of traditional state assignment methods in specific computational scenarios.
• To provide a robust protocol for symmetry assignment of vibrational states.

Main Methods:

• Utilizing Smolyak quadrature for efficient calculation of multidimensional overlap integrals.
• Developing a general protocol for symmetry assignment of vibrational states.
• Performing extensive benchmark calculations using explicitly correlated coupled-cluster theory for potential energy surfaces.

Main Results:

• Demonstrated an efficient route to overcome challenges in vibrational state assignment.
• Successfully applied the method to fundamental modes and overtones of chloromethane (CH3Cl) in various coordinate systems.
• Provided new reference data for the linear CNNC molecule, previously lacking extensive data.

Conclusions:

• The proposed method offers a reliable solution for vibrational state assignment in complex quantum chemical calculations.
• The developed protocol ensures accurate analysis of infrared spectra, even in challenging cases.
• This work advances computational spectroscopy by providing a general and efficient assignment strategy.