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Triple excitations in nuclear-electronic orbital coupled cluster theory for multiple quantum protons
Rowan J Goudy1, Fabijan Pavošević2, Sharon Hammes-Schiffer1
1Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
The Journal of Chemical Physics
|December 9, 2025
Summary
The nuclear-electronic orbital (NEO) framework treats nuclei quantum mechanically, enabling efficient inclusion of nuclear quantum effects. NEO coupled cluster methods with triple excitations accurately calculate molecular properties, including proton affinities.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
- Molecular Physics
Background:
- The nuclear-electronic orbital (NEO) framework enables quantum mechanical treatment of nuclei alongside electrons.
- This approach facilitates the inclusion of nuclear quantum effects, such as anharmonic zero-point energy, in quantum chemical calculations.
- NEO coupled cluster (NEO-CC) methods are effective for accurate ground-state properties of molecular systems.
Purpose of the Study:
- To investigate the inclusion of triple excitations in NEO-CC methods for systems with multiple quantum protons.
- To compare full and perturbative treatments of electron-electron-proton and electron-proton-proton triple excitations.
- To assess the accuracy and efficiency of NEO-CC methods for calculating molecular properties.
Main Methods:
- Exploration of full and perturbative treatments of triple excitations within the NEO-CC framework.
- Application of the NEO-CCSD(T) method, incorporating various triple excitations.
- Utilizing complete basis set extrapolation for proton affinity calculations.
- Single-point energy calculations for protonated water tetramers.
Main Results:
- The perturbative treatment of triple excitations shows quantitative agreement with the full treatment for proton affinity calculations.
- The perturbative approach is significantly more computationally efficient, particularly for systems with multiple quantum protons.
- The NEO-CCSD(T) method accurately reproduces experimental proton affinities within experimental uncertainty.
- The method effectively incorporates anharmonic zero-point energy in complex systems like protonated water tetramers.
Conclusions:
- NEO-CC methods with triple excitations provide an accurate and computationally practical approach for incorporating nuclear quantum effects.
- The perturbative treatment of triple excitations offers a significant computational advantage.
- NEO-CC methods can serve as benchmarks for lower-level NEO methods in studying molecular systems.
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