Time-dependent density functional methods for Raman spectra in open-shell systems.

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• Accurate simulation of molecular spectra is crucial for understanding chemical systems.
• Existing methods often struggle with open-shell molecular systems, limiting their applicability.
• Time-dependent density functional theory (TD-DFT) offers a promising avenue for spectral calculations.

Purpose of the Study:

• To implement and validate a TD-DFT linear response module in NWChem for unrestricted calculations.
• To apply the new module for calculating resonant Raman spectra in open-shell molecular systems.
• To introduce a divide-and-conquer approach for polarizability calculations in larger systems.

Main Methods:

• Implementation of a TD-DFT linear response module for unrestricted DFT in NWChem.
• Application of the short-time approximation for resonant Raman spectra calculation.
• Utilizing a divide-and-conquer method for polarizability evaluation in large systems.

Main Results:

• Successful simulation of resonant Raman spectra for various doublet organic radicals and a metal complex.
• Validation of the implemented tool against experimental data, showing good agreement for most spectral features.
• Identification of factors contributing to deviations, including theoretical approximations and experimental conditions.

Conclusions:

• The developed TD-DFT module provides a valuable tool for studying resonant Raman spectra of open-shell systems.
• The method's accuracy is comparable to that for closed-shell systems, with limitations understood.
• Further refinements in theoretical approximations and inclusion of experimental factors could improve predictive power.