# Licensing & acknowledgements

qc-rs is **free software**, and it stands on the shoulders of established scientific libraries. This appendix
summarizes the license and credits the projects qc-rs builds on. The authoritative developer policy is
[`docs/licensing.md`](https://github.com/qclo/qc_rs/blob/main/docs/licensing.md).

## The license

qc-rs is licensed under the **GNU Lesser General Public License v3.0 or later** (`LGPL-3.0-or-later`; full
text in `COPYING.LESSER` + `COPYING`). In plain terms: you can use it freely — including in commercial and
closed-source work that *links* it — and modifications to qc-rs itself remain open. LGPL matches the
quantum-chemistry ecosystem (Psi4, Dalton, PCMSolver, the Grimme tools, libxc) and, importantly, lets qc-rs
**faithfully port other LGPL/GPL-compatible reference implementations** into Rust while staying open and
linkable.

## What qc-rs builds on

Two external C/Fortran libraries are the hand-written foundations qc-rs links (not AI-authored — the
[exceptions](../00-intro/what-is-qc-rs.md) to its vibe-coded development):

- **[libcint](https://github.com/sunqm/libcint)** (Dr. Qiming Sun) — the Gaussian-integral engine at the
  computational heart of every SCF.
- **[libxc](https://gitlab.com/libxc/libxc)** (Miguel A. L. Marques, Susi Lehtola, and collaborators) — the
  exchange–correlation functional library behind the DFT (`xc=`) methods.

And several reference implementations are **faithfully ported into Rust** (with attribution, reproducing their
numbers):

| qc-rs component | ported from | upstream license |
|---|---|---|
| `pcm-core` (PCM solvation) | [PCMSolver](https://github.com/PCMSolver/pcmsolver) | LGPL-3.0-or-later |
| `qc-disp` (DFT-D3/D4) | Grimme group `s-dftd3` / `dftd4` / `mctc-lib` | LGPL-3.0-or-later |
| GPU kernels (optional `cuda`) | [gpu4pyscf](https://github.com/pyscf/gpu4pyscf) `gint`/`gvhf-rys`/`gdft` | Apache-2.0 (vendored) |

Geometry optimization uses **[geomeTRIC](https://github.com/leeping/geomeTRIC)** (Lee-Ping Wang).

## Clean-room re-implementation (the Multiwfn case)

The [molecular-properties suite](../20-guide/properties/index.md) reproduces *Multiwfn-class* analyses, but
Multiwfn's license is **incompatible with LGPL** (it adds restrictions LGPL forbids). So qc-rs does **not**
port Multiwfn's source. Instead each analysis is **re-implemented clean-room from the published method
papers** — algorithms and equations are not copyrightable, only code is — using Multiwfn (or PySCF) only as a
**black-box** to validate the numbers, and citing the method papers. This is why the manual verifies every
property number against an independent reference.

:::{note} Why this matters to you as a user
Nothing here restricts *using* qc-rs — it is LGPL, so run it for anything, including commercial work. The
policy above governs how the *code* is written and what it may incorporate. It also explains a design value
you see throughout the manual: **numbers are validated against trusted references, never guessed.**
:::

## Acknowledgements

qc-rs gratefully acknowledges libcint (Qiming Sun), libxc (Marques, Lehtola *et al.*), PCMSolver, the Grimme
group (DFT-D3/D4), gpu4pyscf, geomeTRIC (Lee-Ping Wang), **Multiwfn (Tian Lu — the analysis suite's feature
map and black-box validation reference)**, and PySCF (used as a validation reference). Full attributions
live in each crate/file header and the repository `README.md` Acknowledgements.