What is Newton-X?
Newton-X is an open-source software platform for mixed quantum–classical dynamics (MQCD) simulations, used to model excited-state molecular processes in which nuclear motion is treated classically while electronic states evolve quantum mechanically, with nonadiabatic transitions. Surface hopping is a popular example of this class of methods.
Newton-X 26 supports end-to-end workflows, from initial-condition generation and spectra to trajectory propagation and post-processing/archiving.
How can I install Newton-X?
Newton-X is distributed with pre-compiled binaries. For complete setup instructions—including requirements, environment variables, and how to connect Newton-X to your electronic-structure package—please consult the documentation.
Is Newton-X free to use?
Yes. Newton-X is free and open-source software under the GNU General Public License (GPL) v3.0 or later. You may use and modify it freely; if you distribute modified versions, you must provide the corresponding source under the same license terms.
What should I do if I find issues with Newton-X?
If you encounter a problem (unexpected behavior, suspected bug, or installation issue), please report it through the Newton-X support group, which is the main entry point for user support and bug reports.
Please, include your Newton-X version, a short description of what you expected vs. what happened, and any relevant input/output files or log excerpts.
Can I contribute to Newton-X?
Yes. Development takes place on GitLab, and you are welcome to contribute by cloning the repository and working from there.
If you would like a change to be integrated into the official distribution branch, please contact the Newton-X team via info@newtonx.org to discuss the contribution workflow (scope, coding style, testing, and review).
Which MQCD method should I choose (surface hopping vs Ehrenfest/SLED vs AIMS)?
For most applications, surface hopping is the practical default and the usual starting point in Newton-X.
Choose AIMS when you need more explicit quantum features (e.g., advanced decoherence treatment).
Choose Ehrenfest with SLED when you specifically want to explore mean-field dynamics with built-in decoherence; in Newton-X 26, this option is experimental and should be used with care and validated against benchmarks.
Check this paper (10.26434/chemrxiv.15000086/v1) for a discussion on MQCD methods.
Which electronic structure method should I use?
It depends on the type of excited-state process you are modeling and on the electronic-structure character (single-reference vs. multireference) across the relevant geometries.
Photophysics in large systems (excited-state relaxation without major bond rearrangement) is often treated with single-reference methods, most commonly TDDFT (or closely related linear-response approaches). The attraction is simple: reasonable cost and robustness for big molecules, where you need many trajectories.
Photochemistry with bond breaking/formation (especially in small to medium systems) more often requires multireference methods to describe changing electronic character and near-degeneracies. In practice, this points to CASPT2 for dynamics-level calculations.
For a focused discussion on electronic structure choice in surface-hopping contexts, see: 10.1021/acs.jctc.5c01529.
Does Newton-X support intersystem crossing (spin–orbit driven dynamics)?
Not at present. Intersystem crossing (ISC) dynamics is not available in this version of Newton-X.
Newton-X can compute spin–orbit couplings (SOC) within linear-response TDDFT workflows (via PySOC) and use them for spectral simulations, including phosphorescence spectra. However, these SOC capabilities are not currently used to propagate spin-changing nonadiabatic dynamics.
Where are the older (legacy) Newton-X versions/executables?
Legacy Newton-X executables are still distributed (within NX-CS) to support the reproducibility of older inputs and published results. They are frozen: they will not be updated and are not actively supported.
For new simulations, users are strongly encouraged to migrate to the Newton-X 26 workflows and modules.
How should I cite Newton-X?
Please cite Newton-X as:
Barbatti, M.; Bondanza, M.; Crespo-Otero, R.; Demoulin, B.; Dral, P. O.; Granucci, G.; Kossoski, F.; Lischka, H.; Mennucci, B.; Mukherjee, S.; Pederzoli, M.; Persico, M.; Pinheiro Jr, M.; Pittner, J.; Plasser, F.; Sangiogo Gil, E.; Stojanovic, L. Newton-X Platform: New Software Developments for Surface Hopping and Nuclear Ensembles. J. Chem. Theory Comput. 2022, 18, 6851–6865. 10.1021/acs.jctc.2c00804
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References for specific methods, algorithms, and third-party programs used within Newton-X are provided in the documentation.