xcx

Exchange–correlation (XC) functionals for density-functional theory (DFT) — a pure-Rust, libxc-compatible reimplementation with no C dependency.

Given a density (and, per functional, its gradient / kinetic energy density), xcx returns the XC energy per particle, its first and second derivatives (vxc and fxc), plus metadata (family, requirements, exact-exchange fraction, range-separation and VV10 parameters). Each functional is one scalar energy expression; derivatives come from forward-mode automatic differentiation, so they are correct by construction.

xcx maps `(rho, sigma, tau[, lapl]) → energy density + derivatives + metadata

• linear mixing` and nothing else: no grids, AO evaluation, SCF, or dispersion.

See docs/api-convention.md for the frozen data/ABI contract and the full scope fence. Licensed under MPL-2.0 (matching upstream libxc).

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

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))?;
// out.exc[i] = energy per particle; out.vrho[i] = ∂(n·ε)/∂n
# Ok::<(), xcx::XcError>(())