Molecular Vibrational Frequencies within the Nuclear-Electronic Orbital Framework.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Calculating vibrational frequencies is crucial for comparing theoretical models to experimental data in quantum chemistry.
• The nuclear-electronic orbital (NEO) method treats certain nuclei, like protons, quantum mechanically, but faces challenges in calculating vibrational frequencies involving both quantum and classical nuclei due to the Born-Oppenheimer approximation.

Purpose of the Study:

• To develop and implement an effective strategy for calculating the vibrational frequencies of entire molecules within the NEO framework.
• To address the limitations of the standard NEO approach in handling vibrational modes composed of both quantum and classical nuclei.

Main Methods:

• The study devises and implements a strategy involving the diagonalization of an extended NEO Hessian.
• This extended Hessian incorporates expectation values of quantum nuclei and coordinates of classical nuclei.
• The method utilizes input from multicomponent time-dependent density functional theory (NEO-TDDFT).

Main Results:

• The new NEO-DFT(V) approach accurately calculates vibrational frequencies for molecular systems.
• The method effectively incorporates significant anharmonic effects in the calculations.
• Demonstrates the successful application to molecular systems.

Conclusions:

• The developed theoretical formulation provides an effective strategy for calculating vibrational frequencies in multicomponent quantum chemistry.
• This advancement accurately accounts for anharmonic effects, improving theoretical predictions.
• Opens new avenues for research in multicomponent quantum chemistry and molecular spectroscopy.