Create README.md

README.md

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+---
+license: "cc-by-nc-sa-4.0"
+tags:
+ - gas-surface-interaction
+ - normalizing-flow
+ - real-nvp
+ - variational-autoencoder
+ - molecular-dynamics
+ - piclas
+ - dsmc
+ - rarefied-gas-dynamics
+ - vleo
+library_name: "piclas"
+---
+
+# PICLas-ML/GSI - ML Gas-Surface Interaction Models
+
+Machine-learned surface scattering models for [PICLas](https://github.com/piclas-framework/piclas),
+an open-source 3D particle-based kinetic simulation framework for plasma dynamics and rarefied
+gas flows.
+
+## Models in Repo
+
+This repo contains two model architectures for calculating the scattering of atomic oxygen on an aluminum oxide surface Al₂O₃:
+
+- **cRealNVP** - a conditional real-valued non-volume-preserving flow, using a detailed-balance
+  loss as a physics constraint
+- **cVAE** - a conditional variational autoencoder
+
+| File | Architecture | Incident Species → Surface → Reflected Species  |
+|---|---|---|
+| `cRealNVP_MDclassic_O-Al2O3-O.h5` | Conditional RealNVP (normalizing flow) | O → Al₂O₃ → O |
+| `cVAE_MDclassic_O-Al2O3-O.h5` | Conditional variational autoencoder | O → Al₂O₃ → O |
+
+**File naming convention:** `<architecture>_<data source>_<species>.h5`
+- 1st part — model architecture: `cVAE` or `cRealNVP`
+- 2nd part — source of the non-equilibrium data: MD simulation with a classical potential
+- 3rd part — atomic oxygen impacting an aluminum oxide surface; outgoing species is atomic oxygen
+
+**Performance comparison:**
+
+| | cRealNVP | cVAE |
+|---|---|---|
+| Accuracy (thermal → 11,000 m/s) | Better overall | Comparable till 10,000 m/s |
+| Equilibrium temperature | Converges correctly | Does not converge correctly |
+| Sampling speed | Slower | Much faster |
+
+## Method
+
+These models were developed as a collaboration within the [Collaborative Research Center 1667 ATLAS](https://www.sfb1667.uni-stuttgart.de/)
+at the University of Stuttgart. A detailed description of the models and the data is given in
+[arXiv:2606.31928](https://arxiv.org/abs/2606.31928) - *Conditional Normalizing Flow for
+Gas-Surface Scattering from Thermal to Hypersonic Velocities*.
+
+**Data**
+
+Two training data sets are used, both for atomic oxygen impacting an aluminum oxide (Al₂O₃)
+surface:
+- Non-equilibrium data generated from MD simulations
+- Equilibrium data generated from Maxwell flux distribution
+
+*Non-Equilibrium Data:*
+- Atomic oxygen impacts on an aluminum oxide surface are simulated with molecular dynamics
+  using classical potentials
+- Incident velocity magnitudes: 2,000-10,000 m/s
+- Polar angles: 0°-80°
+
+*Equlibrium:*
+- If the gas is in equilibrium with the wall, an incident Maxwell flux must be reflected as the same Maxwell flux
+- Maxwell flux samples are added to the training data to cover the equilibrium regime
+
+**Model**
+
+- In PICLas, scattering is represented through a scattering kernel, i.e., the conditional probability distribution P(v_i → v_r) describing how an incoming velocity transitions to a reflected one.
+- To learn this kernel from data, two generative ML models (cVAE and cRealNVP) are trained on the data
+
+## Intended use
+
+- Load into PICLas as an ML surrogate GSI scattering model to sample scattered particle velocities
+- **Valid for:** O → Al₂O₃ collisions, from the thermal regime up to 11,000 m/s impact velocity
+- **Not valid for:** other species/materials, conditions far outside the training data, or
+  chemistry beyond what the classical MD potential captures
+
+## Usage in PICLas
+
+Example entry in `parameter.ini`:
+
+```
+Part-Boundary1-SurfaceModel = 0 ! only Scattering: 0
+Part-Boundary1-SurfaceModelScattering = 3   ! cVAE: 2, cRealNVP: 3
+SurfaceScattering-NumOfMLs = 1
+SurfaceScattering-ML1-NumOfBoundaries = 1
+SurfaceScattering-ML1-Boundaries = (/1/)
+SurfaceScattering-ML1-File = https://huggingface.co/PICLas-ML/GSI/resolve/main/cRealNVP_MDclassic_O-Al2O3-O.h5
+```
+
+The model is downloaded and cached under `piclas-ml/gsi/` on first use.
+
+For further information, visit the [PICLas documentation](https://piclas.readthedocs.io/en/latest/).
+
+## Citation
+
+If your research leads to a publication, please cite the models using:
+
+```bibtex
+@misc{schütte2026conditionalnormalizingflowgassurface,
+      title={Conditional Normalizing Flow for Gas-Surface Scattering from Thermal to Hypersonic Velocities}, 
+      author={Miklas Schütte and Stephen Hocker and Hansjörg Lipp and Johannes Roth and Stefanos Fasoulas and Marcel Pfeiffer},
+      year={2026},
+      eprint={2606.31928},
+      archivePrefix={arXiv},
+      primaryClass={physics.comp-ph},
+      url={https://arxiv.org/abs/2606.31928}, 
+}
+```