# integral-core
Layer-1/2 integral engines and operator layer for
[integral](https://github.com/nmrtist/integral), a native-Rust library for Gaussian
integrals in quantum mechanics.
This crate hosts:
- **One-electron Obara–Saika engine** — overlap, kinetic, nuclear attraction, multipole.
- **OS/HGP two-electron engine** — flat-array HRR + m-marching VRR + reusable arena.
- **Rys two-electron engine** — small-footprint quadrature path for all angular momenta.
- **Operator DSL** — one-electron integrands as polynomials in `r` and `p = −i∇`.
- **Geometric first derivatives** — via the Gaussian center-derivative relation.
Most users want the higher-level [`integral`](https://crates.io/crates/integral) driver
crate rather than this one. `#![forbid(unsafe_code)]`, pure stable Rust.
Licensed under Apache-2.0.