Cargo features

The facade crate xtalforge is the crate applications depend on. Its features are additive: enabling one never changes results, only which optional capabilities are compiled. Every domain crate carries the same switches so a downstream crate can be built directly with or without them.

Feature	Default	Domains	Effect
parallel	yes	core, io, symmetry	Reserves Rayon for the parallel algorithms. The current algorithms are single-threaded and deterministic; the feature exists so builds can already opt out with --no-default-features.
sci-form	no	force fields	Links the published MIT crate sci-form 2.0.0 for in-process GFN0/GFN1/GFN2 (Task 4.10). The methods are molecular only; GFN2 returns forces and the registry lists gfn0, gfn1 and gfn2.
ml	no	force fields	Reserved for machine-learned potentials (Task 8.4). The decision is recorded in docs/algorithms/ml-backend.md; currently a no-op.
external	no	force fields	Reserved; the runtime engine probes and input writers for DFTB+, xTB and ORCA are always compiled and need no feature (Task 4.9).
mof-data	no	MOF builder	Reserved for a future full MOF bundle; the curated six-topology/eight-block subset is always embedded (Tasks 6.11-6.16).

Building with and without defaults

cargo build -p xtalforge                     # default features (parallel)
cargo build -p xtalforge --no-default-features
cargo build -p xtalforge --features sci-form
cargo build -p xtalforge --all-features

Rules for new features

1. Default features must depend only on pure Rust crates (AGENTS.md rule 3). Anything that needs a native library, a subprocess or a large embedded data set is opt-in.
2. A feature is documented in the facade crate docs and in this page in the same commit that introduces it.
3. The WASM build keeps heavy and external features disabled by default; the parity matrix records the exception when a capability cannot exist in the browser.
4. Features stay additive. Two features enabled together must behave like the union of their capabilities, never like a different code path that changes numeric output.
5. The default WASM package has a 5120 KiB budget, checked by scripts/check-wasm-size.sh. The hello-world build was about 41 KiB; the Phase 1 readers and builders brought it to about 438 KiB, symmetry to about 813 KiB, and the Phase 3-7 surfaces (including the polymer/embedding code) and the agent session took the release package to 3943 KiB at 1.1.0. The Phase 9 core features took it to 4603 KiB at 1.2.0 and the MOF platform to 4936 KiB at 1.3.0, inside the 5120 KiB budget.

Threaded WebAssembly

The default xtalforge-wasm package is single-threaded. A threaded build needs a shared WebAssembly.Memory, SharedArrayBuffer and cross-origin isolation headers from the host page; without them the module cannot start. The Rust parallel feature is not enabled in the published WASM build, and enabling it never changes a result: every parallel reduction uses a fixed order, so the single-threaded and threaded paths agree.