Multicomponent Quantum Chemistry: Integrating Electronic and Nuclear Quantum Effects via the Nuclear-Electronic...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multicomponent quantum chemistry methods treat multiple particle types quantum mechanically.
• The nuclear-electronic orbital (NEO) approach quantizes specific nuclei alongside electrons.
• This allows direct inclusion of nuclear quantum effects like delocalization and tunneling.

Purpose of the Study:

• To introduce and detail the nuclear-electronic orbital (NEO) approach in quantum chemistry.
• To highlight the impact of nuclear quantum and non-Born-Oppenheimer effects on molecular properties.
• To review various NEO-based methods for calculating ground and excited states.

Main Methods:

• Nuclear-Electronic Orbital (NEO) methods, including NEO-DFT, NEO-TDDFT, NEO-CCSD, and NEO-EOM-CCSD.
• Orbital-optimized perturbation theory (NEO-OOMP2) for improved efficiency.
• Multireference methods like NEO-ΔSCF, NEO-CASSCF, and nonorthogonal configuration interaction.

Main Results:

• NEO methods accurately incorporate nuclear quantum effects (delocalization, zero-point energy, tunneling) and non-Born-Oppenheimer effects.
• NEO-DFT and NEO-TDDFT offer a balance of efficiency and accuracy for ground and excited states.
• Wave function based NEO methods (e.g., NEO-CCSD) provide high accuracy, while NEO-OOMP2 offers a computationally cheaper alternative.

Conclusions:

• The NEO approach provides an accessible and efficient way to include nuclear quantum and non-Born-Oppenheimer effects in quantum chemistry.
• These effects significantly influence molecular geometries, vibrational frequencies, reaction pathways, and dynamics.
• NEO methods are crucial for accurately describing phenomena like hydrogen tunneling and for studying diverse multicomponent systems.