# UFP Documentation

UFP is a Torch-first library for building, fitting, and using interatomic
potentials from composable energy terms. It keeps ASE and simulation-engine
objects at the boundary, then evaluates models on normalized torch tensors.

The documentation is organized around the main decisions a user or developer
needs to make:

```{toctree}
:maxdepth: 2

getting-started
concepts
model-terms
adding-terms
fitting
uncertainty
coefficient-workflows
workflow-management
runtime-backends
examples
architecture
benchmarks
api
development
```

## What UFP Provides

- A shared `UFPInput` representation for atoms, cells, periodic flags, and
  neighbor lists.
- Additive `UFPModel` objects built from one-body, pair, spline, three-body,
  four-body, repulsion, and alchemical terms.
- Two fitting paths: direct linear least squares for coefficient-linear models
  and PyTorch training for differentiable models.
- ASE calculator support, metatomic wrappers, torch-sim wrappers, and optional
  native C++/CUDA acceleration for selected three-body paths.

## Units

UFP does not impose a unit registry. Most examples and ASE workflows use
angstroms for length and electron volts for energy. Forces then have units of
energy per length, typically eV/angstrom. Keep units consistent across input
structures, fitted coefficients, and labels.