General formulation of vibronic spectroscopy in internal coordinates

Area of Science:

• Computational Chemistry
• Spectroscopy
• Quantum Mechanics

Background:

• Vibronic effects are crucial for understanding molecular spectroscopy.
• Accurate calculations require appropriate coordinate systems.
• Existing methods have limitations in handling molecular flexibility.

Purpose of the Study:

• To extend a computational platform for vibronic effect calculations.
• To incorporate various internal coordinate systems.
• To improve the accuracy and applicability of vibronic spectroscopy simulations.

Main Methods:

• Implementation of analytical derivatives for internal coordinates.
• Support for both vertical and adiabatic models, including mode mixing and Herzberg-Teller contributions.
• Automated generation of non-redundant internal coordinate sets from molecular topology.

Main Results:

• The platform now supports diverse internal coordinates for vibronic calculations.
• Cartesian and internal coordinates show comparable results for rigid systems.
• Delocalized internal coordinates are superior for flexible systems with significant geometry changes during electronic transitions.

Conclusions:

• The enhanced platform provides a more versatile and accurate tool for vibronic spectroscopy.
• The choice of coordinate system significantly impacts results for flexible molecules.
• The developed methods facilitate deeper understanding of electronic transitions in complex molecular systems.