xcx

Exchange–correlation (XC) functionals for density-functional theory (DFT) in pure Rust, built around automatic differentiation — 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, VV10, and PT2 parameters). Each functional is one scalar energy expression; all derivatives come from forward-mode automatic differentiation, so they are correct by construction.

xcx keeps libxc ids and conventions for drop-in interoperability (golden-verified to ≤ 1e-10 where the two overlap), and goes beyond it: a first-class double-hybrid family (B2PLYP, revDSD-PBEP86-D4, PWPB95, ωB97M(2)) with structured PT2/CAM metadata that libxc does not ship.

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 per file: original xcx code under MIT OR Apache-2.0; code derived from libxc under MPL-2.0 (see NOTICE).

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>(())