Built-in differentiable floor: self-consistent-charge TB with Mermin free energy (finite electronic temperature keeps occupations smooth ⇒ ladder stays differentiable through band crossings). Gradients: Hellmann-Feynman for dE/dθ, IFT through the SCC fixed point. Build on tang-expr so the electronic rung joins the same autodiff substrate as the constraint solver and phyz. Validate with the atoms::fd oracle pattern.
Accept: ethylene gap + gap gradient vs finite difference; WASM-able for small systems.
Blocked by #501.
Built-in differentiable floor: self-consistent-charge TB with Mermin free energy (finite electronic temperature keeps occupations smooth ⇒ ladder stays differentiable through band crossings). Gradients: Hellmann-Feynman for dE/dθ, IFT through the SCC fixed point. Build on tang-expr so the electronic rung joins the same autodiff substrate as the constraint solver and phyz. Validate with the atoms::fd oracle pattern.
Accept: ethylene gap + gap gradient vs finite difference; WASM-able for small systems.
Blocked by #501.