Glossary
This short glossary connects the scientific and software vocabulary used in this manual. It is an orientation aid; method definitions and API contracts remain in their authoritative references.
| Term | Meaning in this manual |
|---|---|
| AO / MO | Atomic orbital / molecular orbital. Basis functions centered on atoms represent the molecular orbitals used by HF and MP2. |
| Basis set | A finite collection of functions used to represent electronic orbitals. STO-3G is a small basis suitable for the first example, not a general accuracy guarantee. |
| Bohr / Angstrom | Length units. Bohr is the atomic unit of length; Angstrom is commonly used for molecular coordinates. RustiQ converts input coordinates to Bohr for calculations. |
| Hartree | The atomic unit of energy used in RustiQ’s reported energies. |
| HF / RHF / UHF | Hartree–Fock / restricted HF / unrestricted HF. RHF pairs electrons in shared spatial orbitals; UHF permits different spatial orbitals for alpha and beta spins. |
| SCF | Self-consistent field: the iterative procedure for solving the HF equations. Meeting its convergence criterion does not prove chemical accuracy. |
| DIIS | Direct inversion in the iterative subspace: an acceleration technique for SCF iterations. |
| MP2 | Second-order Møller–Plesset perturbation theory: a correlation correction built from an HF reference. |
| Multiplicity | 2S + 1 for spin quantum number S. Singlet and doublet mean multiplicities one and two. Charge and multiplicity specify the intended molecular state. |
| Spin contamination | Deviation of UHF’s spin expectation value from the value for the intended pure spin state. |
| ERI | Electron-repulsion integral: a contribution describing the Coulomb interaction between electrons in the chosen basis. |
| AO ERI | Atomic-orbital electron-repulsion integral. RustiQ’s optional local cache stores these deterministic integrals, not HF or MP2 results. |
| Orthogonalization | Construction of an orthonormal orbital space from overlapping basis functions. Near-dependent directions may be removed. |
| Frozen orbitals | Occupied orbitals excluded from the MP2 correlation calculation. |
| ECP | Effective core potential: a model replacing core electrons and their nuclear interaction; currently unsupported by RustiQ’s basis loader. |
| Rust / Cargo | Rust is the implementation language; Cargo builds Rust packages and runs their tests and examples. Users can run an installed CLI without writing Rust. |
| Crate / workspace | A Rust package’s library or executable unit / a group of packages built together. RustiQ separates its CLI and scientific library. |
| CLI / API | Command-line interface for terminal users / application programming interface for code using the library. |
| rustdoc / mdBook | Rust’s API documentation generator / the tool that builds this user and developer manual. |
| TOML / Nickel | Calculation input languages. TOML provides declarative settings; Nickel also supports imports and computed studies and owns input defaults and validation. |
| JSON / schema | A structured data format / the contract describing valid data. Existing versioned schemas define RustiQ’s machine-readable results. |
| Nix / flake | A tool for reproducible packages and environments / this repository’s project definition, with dependencies locked by flake.lock. Nix does not require NixOS. |
| Dev Container | A prepared Linux development environment running through Docker, with editor integration. |
| Cache / portable artifact | Disposable local data used to avoid repeated work / an intentional scientific snapshot. .rustiq bundles can be written, reused, and inspected through the CLI or Rust APIs. |
| Validation / tolerance | Comparison against independent evidence / the allowed difference in a particular numerical check. Passing selected checks does not validate every calculation. |
| Contracted Gaussian | A basis function formed by a weighted sum of primitive Gaussian functions that share a center and angular momentum. |
| Primitive Gaussian | A Gaussian function used as a component of a contracted basis function. |
| Occupied / virtual orbital | An orbital assigned electrons in the reference determinant / an unoccupied orbital available as an excitation destination. |
| Fock matrix | The effective one-electron matrix built from the current electron density in Hartree–Fock. |
| Density matrix | A matrix encoding which molecular orbitals are occupied and their contributions in the atomic-orbital basis. |
| Overlap matrix | The matrix of pairwise integrals between basis functions; its off-diagonal values reflect that these functions need not be orthogonal. |
| Cartesian / spherical Gaussian | Two ways to represent the angular part of Gaussian basis functions. Their function counts differ for d and higher angular momentum. |
| Correlation energy | The energy correction beyond the chosen HF reference; RustiQ reports MP2 correlation separately from the HF and total energies. |
| Linear dependence | Near-redundancy among basis functions, which can make the overlap matrix numerically singular. RustiQ may discard the affected directions. |