---
name: feos-python
description: Helps write Python code against the feos thermodynamics package (pip install feos) — an equation-of-state and classical DFT toolkit (PC-SAFT, ePC-SAFT, GC-PC-SAFT, PETS, UV-theory, SAFT-VR-Mie, SAFT-VR-Q-Mie, Peng-Robinson, multiparameter/reference EOS). Use this whenever the user is working with the `feos` Python package: building an EquationOfState or HelmholtzEnergyFunctional, defining or loading PureRecord/Parameters, working with si_units/SIObject dimensioned quantities, computing State properties, phase equilibria (bubble/dew/flash/critical points), phase diagrams, parameter fitting/regression against experimental data, or DFT calculations (surface tension, adsorption, pores, pair correlation). Trigger even if the user doesn't say "feos" explicitly but mentions PC-SAFT, SAFT equations of state, or thermodynamic property/phase-equilibrium calculations in Python that look like they involve this package.
---

feos is a Rust thermodynamics/classical-DFT library exposed to Python via PyO3 (pip install feos). This skill is a lookup reference for the Python API, split into files loaded on demand — don't try to hold the whole API in context at once, jump to the file that matches the task.

The Python-facing API is generated from Rust and evolves; IDE stub files (feos.pyi) can lag behind the installed version. If something in these reference files doesn't match what you observe, or you need to check a detail not covered here, the installed package is always the authoritative source — introspect it directly:

import feos, inspect
inspect.signature(feos.EquationOfState.pcsaft)   # exact call signature
help(feos.EquationOfState.pcsaft)                 # full docstring, if present
[x for x in dir(feos) if not x.startswith('_')]   # what's available at all

When a parameter object's exact required fields are unclear, the fastest way to find out is to try building it with too few/wrong fields — the error message names the missing field (see references/models.md for why this works and how it was used to build that file).

Every task follows the same skeleton, regardless of model:

1. Build one or more feos.PureRecords (parameters.md) — or load Parameters/ GcParameters straight from a file.
2. Turn them into a feos.Parameters (or GcParameters) object.
3. Construct a model: feos.EquationOfState.<model>(parameters, ...) for bulk thermodynamics, or feos.HelmholtzEnergyFunctional.<model>(parameters, fmt_version, ...) for classical DFT (models.md).
4. Do the actual task: build a State and read properties, run a phase-equilibrium/ phase-diagram calculation, fit parameters against data, or run a DFT calculation — each with its own reference file under references/tasks/.

All physical inputs/outputs are si_units.SIObjects, not plain floats — read references/units.md first if this is unfamiliar; every other reference file assumes it.