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The Importance of Going beyond the Independent Atom Model When Predicting UED Signals from Simulations
Lewis Hutton1, Andrés Moreno Carrascosa1, Mats Simmermacher1
1Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QZ, U.K.
Journal of Chemical Theory and Computation
|June 10, 2026
Summary
The independent atom model (IAM) provides a basic prediction for ultrafast electron diffraction (UED) signals but fails to capture crucial details. Advanced ab initio calculations are essential for accurate analysis of chemical dynamics.
Area of Science:
- Physical Chemistry
- Chemical Physics
- Computational Chemistry
Background:
- Ultrafast electron diffraction (UED) is a powerful technique for studying molecular dynamics.
- The independent atom model (IAM) is often used for initial analysis of UED signals.
- Accurate theoretical prediction of UED signals requires understanding the necessary level of theory.
Purpose of the Study:
- To investigate the theoretical level required for accurate prediction of UED signals in photoexcited γ-butyrolactone.
- To compare IAM predictions with ab initio calculations for UED signals.
- To assess the impact of electronic structure and dynamics on UED signal prediction.
Main Methods:
- Calculated total isotropic diffraction signals using IAM and ab initio electronic wave functions.
- Benchmarked results at equilibrium geometry, along a photochemical reaction coordinate, and during nonadiabatic dynamics simulations (fewest-switches surface-hopping).
- Analyzed deviations between IAM and ab initio results, focusing on intensity and position of scattering features.
Main Results:
- IAM qualitatively captures time-resolved UED signals but shows significant deviations from ab initio results.
- Errors in IAM persist even when considering nuclear wavepacket dynamics.
- Deviations are most pronounced in scattering feature intensity at small to medium momentum transfer, impacting lifetime determination.
Conclusions:
- The IAM is insufficient for accurate quantitative analysis of UED signals, particularly for dynamics.
- Errors from IAM can lead to inaccuracies when inverting scattering data.
- Ab initio calculations are crucial to incorporate effects of chemical bonding, electronic excitation, and ionization for reliable UED analysis.
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