Molecular Vibrational Frequencies with Multiple Quantum Protons within the Nuclear-Electronic Orbital Framework

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• The nuclear-electronic orbital (NEO) approach treats electrons and nuclei, like protons, at the same quantum mechanical level.
• Proton vibrational excitations are calculable via multicomponent time-dependent density functional theory (NEO-TDDFT) for fixed nuclei.
• The NEO-DFT(V) approach was recently developed for molecular vibrational frequencies involving both classical and quantum nuclei.

Purpose of the Study:

• To devise strategies for extending NEO-TDDFT and NEO-DFT(V) to molecules with multiple quantum protons.
• To enable self-contained, effective, and computationally practical calculations for complex systems.
• To explore the description of collective nuclear motions and anharmonic effects in molecular vibrations.

Main Methods:

• Utilizing multicomponent time-dependent density functional theory (NEO-TDDFT).
• Developing the NEO-DFT(V) approach with an extended NEO Hessian.
• Implementing strategies for handling multiple quantum protons within the NEO framework.

Main Results:

• The NEO-TDDFT method successfully describes vibrational excitations of collective nuclear motions.
• The NEO-DFT(V) approach effectively incorporates significant anharmonic effects, particularly in hydrogen stretching modes.
• Strategies were developed for computationally practical calculations involving multiple quantum protons.

Conclusions:

• The presented theoretical strategies enable advanced multicomponent quantum chemistry applications.
• These methods provide accurate descriptions of molecular vibrations, including anharmonicity.
• The work paves the way for broader applications of quantum mechanical treatments of nuclei and electrons.