Gaussian basis sets

A basis set is a finite collection of functions used to represent molecular orbitals. RustiQ’s Gaussian basis functions are built from Gaussian functions centered on atoms. The chosen basis affects the size of the orbital space and the numerical result. A small basis such as STO-3G is useful for examples but does not guarantee accurate energies or properties.

Offline workflow

Import the checked-in STO-3G basis fixture into RustiQ’s local basis store and run the H₂ example:

cargo run --locked -- basis import tests/data/sto-3g.json
cargo run --locked -- run samples/h2/sto-3g/calculation.toml

The import is local and does not need network access. The test suite exercises the same fixture using an isolated store in tests/cli_samples.rs and basis import/storage behavior in crates/rustiq-core/src/basis/basis_store.rs.

List, import, or remove locally stored bases with:

cargo run --locked -- basis list
cargo run --locked -- basis import tests/data/sto-3g.json
cargo run --locked -- basis remove sto-3g

The default store is in the platform’s application data directory. Set RUSTIQ_DATA_BASIS to choose its exact path, or RUSTIQ_DATA_HOME to choose the data root. The CLI guide in existing workflows lists platform paths and migration notes.

Online workflow

The default CLI build enables the online feature. Use it to list or download bases:

cargo run --locked -- basis list --online
cargo run --locked -- basis download sto-3g

Online commands require network access. A normal run uses the local basis store; automatic downloading can be enabled with --auto-download, disabled with --no-auto-download, or set using RUSTIQ_AUTO_DOWNLOAD. Builds without the online feature can import, list local files, and remove bases but cannot download or query online listings. The commands reflect the CLI tests in tests/cli_samples.rs.

Supported input features

Basis files are imported in the Basis Set Exchange JSON form. A shell groups functions with the same center and angular momentum: \(l=0,1,2\) denote s, p, and d functions. Cartesian and spherical describe two function forms that differ in how angular dependence is represented. For elements used by a molecule, the current loader accepts Gaussian-type and Cartesian Gaussian shells through angular momentum \(l=15\), and spherical Gaussian shells through \(l=2\). It rejects Slater-type shells and effective core potentials (ECPs). Higher angular momentum or other basis-file features should not be assumed to work just because a basis is downloadable. This describes loader support, not numerical validation of all those cases. The checks are in basis.rs.

Basis choice is one part of a calculation’s accuracy. Convergence, independent agreement, basis quality, and physical validity are distinct checks; see scientific scope. See the glossary for shared basis vocabulary.

Next: TOML configuration.