General Perturbative Approach for Spectroscopy, Thermodynamics, and Kinetics: Methodological Background and Benchmark...

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Quantum Chemistry

Background:

• Accurate calculation of molecular vibrational properties is crucial for understanding chemical reactivity and physical behavior.
• Existing perturbative methods often face limitations with singularities, internal rotations, and threshold dependencies.
• Developing robust, automated methods for computing thermodynamic and spectroscopic properties is essential for complex molecular systems.

Purpose of the Study:

• To introduce and validate a general, resonance- and threshold-free second-order perturbative approach for vibrational properties.
• To extend degeneracy corrected second-order perturbation theory (DCPT2) to a hybrid version (HDCPT2) for improved reliability.
• To enable accurate, automated ab initio calculations of thermodynamic and spectroscopic properties for large, biotechnologically relevant molecules.

Main Methods:

• Developed a singularity-avoiding method for anharmonic zero-point vibrational energies.
• Extended DCPT2 to HDCPT2, incorporating an automatic treatment of internal rotations via the hindered-rotor model.
• Reformulated simple perturbation theory (SPT) for consistent treatment of energy minima and transition states.
• Validated the HDCPT2 model by computing anharmonic vibrational frequencies for various molecular systems.

Main Results:

• The HDCPT2 approach provides reliable computations, overcoming limitations of the original DCPT2 formulation.
• The method accurately computes anharmonic vibrational frequencies, achieving accuracy close to established models.
• The overall approach yields thermodynamic and spectroscopic properties with high accuracy (e.g., ~1 kJ/mol for enthalpies).
• The B3LYP/aug-N07D and B2PLYP/aug-cc-pVTZ models show reliability for anharmonic computations.

Conclusions:

• The proposed general perturbative approach offers accurate and automated ab initio calculations of vibrational properties.
• HDCPT2 and associated methods support experimental investigations for large molecular systems without ad hoc scaling.
• The study demonstrates a significant advancement in the computational prediction of molecular thermodynamic and spectroscopic data.