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Reaching precise proton affinities in non-Born-Oppenheimer calculations
Luukas Nikkanen1, Susi Lehtola1
1Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland.
The Journal of Chemical Physics
|April 17, 2026
Summary
This study shows that most protonic basis sets are sufficient for non-Born-Oppenheimer (non-BO) calculations of proton affinities (PAs). Uncontracting electronic basis sets on quantum protons significantly improves convergence for these quantum mechanical models.
Area of Science:
- Quantum Chemistry
- Computational Chemistry
Background:
- Modeling nuclear quantum effects requires advanced methods like the non-Born-Oppenheimer (non-BO) approach.
- This method treats nuclei quantum mechanically alongside electrons, using distinct nuclear and electronic basis sets.
Purpose of the Study:
- Investigate the convergence of non-BO proton affinities (PAs) with respect to protonic and electronic basis sets.
- Determine optimal basis set strategies for accurate non-BO calculations.
Main Methods:
- Performed non-BO density-functional calculations of proton affinities.
- Analyzed convergence behavior using various protonic and electronic basis sets.
- Examined the impact of uncontracting electronic basis sets on quantum protons.
Main Results:
- Most protonic basis sets achieve convergence within 0.1 kcal/mol for PAs.
- Truncation error in non-BO calculations is primarily driven by the electronic basis set.
- Uncontracting electronic basis sets on quantum protons accelerates convergence to the basis set limit.
Conclusions:
- Smaller protonic basis sets may be feasible for non-BO PA calculations.
- Uncontracted electronic basis sets offer higher quality results with minimal computational overhead.
- Specific basis sets like aug-pc-3, when uncontracted, provide converged PAs efficiently.
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