Crate xcx 

xcx — exchange–correlation functionals for DFT (pure-Rust, libxc-compatible)

xcx evaluates exchange–correlation (XC) functionals: given a density (and, depending on the functional, its gradient / kinetic energy density), it returns the XC energy per particle, its first and second derivatives (vxc and fxc), together with metadata (family, requirements, exact-exchange fraction, range-separation and VV10 parameters).

Each functional is written once as a scalar energy expression; derivatives are obtained by forward-mode automatic differentiation, so they are correct by construction. Functional IDs match libxc.

Scope fence

xcx maps (rho, sigma, tau[, lapl]) → energy density + derivatives + metadata + linear mixing and nothing else — no grids, atomic-orbital evaluation, SCF driver, or dispersion. For hybrids and VV10 it exposes the parameters; it does not compute the exact-exchange or nonlocal integrals.

The full, semver-stable contract lives in docs/api-convention.md.

Example

use xcx::{Functional, FunctionalId, Spin, XcInput};

// Spin-unpolarized LDA exchange over three grid points.
let f = Functional::new(FunctionalId::LdaX, Spin::Unpolarized)?;
let rho = [0.1_f64, 0.2, 0.3];
let out = f.eval(rho.len(), &XcInput::lda(&rho))?;

assert_eq!(out.exc.len(), 3); // energy per particle at each point
assert_eq!(out.vrho.len(), 3); // ∂(n·ε_xc)/∂n at each point

Structs

CamParams

Range-separation (CAM) parameters: the host builds short/long-range exact exchange from these. None of the v0.1 functionals are range-separated.

Functional

A ready-to-evaluate functional bound to a spin treatment.

FunctionalInfo

Static metadata describing a functional. See docs/api-convention.md.

HybridInfo

Mixing information for hybrid functionals.

Vv10Params

VV10 nonlocal-correlation parameters. xcx exposes these but never computes the nonlocal integral. None of the v0.1 functionals use VV10.

XcInput

Inputs to a functional evaluation, as borrowed point-major slices.

XcResult

Energy per particle and first derivatives. Vectors are point-major; the ones not produced by a given family are left empty.

Enums

Family

Functional family.

FunctionalId

A functional identifier. Numeric values equal libxc’s for interoperability.

Kind

Whether a functional models exchange, correlation, both, or kinetic energy.

Spin

Spin treatment of the density.

XcError

Errors returned by the public API.