xcx 0.3.1

Exchange–correlation functionals for DFT in pure Rust — autodiff derivatives (vxc/fxc), double hybrids, libxc-interoperable ids
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xcx — exchange–correlation functionals for density-functional theory in pure Rust
Copyright (c) 2026 Jiekang Tian and the xcx authors

==============================================================================
Licensing scheme
==============================================================================

xcx is licensed per file, under one of two schemes. Every Rust source file
carries an SPDX header (and, for functional code, a `Provenance:` tag) stating
which applies:

1.  Original xcx code — the public API and data-layout layer, the automatic-
    differentiation family harnesses, the test suites, and every functional
    tagged `Provenance: clean-room` (implemented from the published
    literature, without reference to the libxc source) — is dual-licensed
    under either of

        * the MIT License (LICENSE-MIT), or
        * the Apache License, Version 2.0 (LICENSE-APACHE),

    at your option. Header: `SPDX-License-Identifier: MIT OR Apache-2.0`.

2.  Code tagged `Provenance: ported-from-libxc` — functional formulas,
    the reduced-variable helper layer, and numerical-stability
    reformulations derived from libxc's Maple/C sources — remains under the
    Mozilla Public License, Version 2.0 (LICENSE-MPL), the license of the
    upstream work. Files of mixed provenance also stay under the MPL-2.0.

The combined SPDX expression for the crate as a whole is therefore
`(MIT OR Apache-2.0) AND MPL-2.0`. Because MPL-2.0 is file-level copyleft,
this has no practical effect on downstream users: you may depend on xcx from
MIT, Apache, BSD, or proprietary projects freely; only modifications to the
MPL-tagged source files themselves must remain available under the MPL.

==============================================================================
Upstream attribution
==============================================================================

Portions of this library are derived from libxc:

    libxc — https://libxc.gitlab.io/
    Copyright (C) the libxc authors, including M. A. L. Marques, X. Andrade,
    S. Lehtola, and M. Oliveira.
    Licensed under the Mozilla Public License, Version 2.0.

==============================================================================
Per-file provenance
==============================================================================

Every functional source file carries one of the following tags in its header:

    Provenance: ported-from-libxc
        Derived from the corresponding libxc Maple/C source. MPL-2.0.

    Provenance: clean-room
        Implemented from the published literature, without reference to the
        libxc source. MIT OR Apache-2.0.

See the individual file headers (SPDX identifier + provenance tag) to
determine the licensing of any given file.

==============================================================================
Contributions
==============================================================================

Unless you explicitly state otherwise, any contribution intentionally
submitted for inclusion in xcx by you shall be licensed per the scheme above
(MIT OR Apache-2.0 for original code; MPL-2.0 for code derived from libxc),
without any additional terms or conditions.