Enhancing the applicability of multicomponent time-dependent density functional theory

Area of Science:

• Quantum Chemistry
• Computational Spectroscopy
• Theoretical Chemistry

Background:

• Time-dependent density functional theory (TDDFT) traditionally treats nuclei classically.
• The nuclear-electronic orbital (NEO) framework allows quantum mechanical treatment of both electrons and nuclei.
• Simultaneous calculation of electronic and vibrational excitations is computationally demanding.

Purpose of the Study:

• To investigate the accuracy of proton vibrational excitation energies calculated using the NEO-TDDFT approach.
• To examine the influence of nuclear and electronic basis sets on these calculations.
• To implement and utilize nuclear transition densities for characterizing vibrational modes.

Main Methods:

• Utilizing the multicomponent extension of TDDFT within the NEO framework.
• Performing quantum mechanical treatment of all electrons and select nuclei (protons).
• Systematically varying protonic and electronic basis sets, including f functions for quantum hydrogen.

Main Results:

• Protonic basis sets with f functions and extensive electronic basis sets yield accurate proton vibrational excitation energies (within ~30 cm^-1 of reference values).
• The NEO-TDDFT method demonstrates effectiveness for both open-shell and closed-shell systems.
• A method for computing and visualizing nuclear transition densities for proton vibrations was successfully implemented.

Conclusions:

• NEO-TDDFT provides accurate vibrational excitation energies by treating protons quantum mechanically.
• The choice of basis sets, particularly for protons, is crucial for achieving high accuracy.
• The developed nuclear transition densities enhance the characterization of vibrational modes and their spatial orientations, aiding in anharmonic effect incorporation.