Function pairs_within 

pub fn pairs_within(
    positions: &[[f32; 3]],
    left: &AtomSelection,
    right: &AtomSelection,
    cutoff: f32,
    backend: SpatialBackend,
    periodic: Option<&PeriodicBox>,
    context: &ExecutionContext,
) -> Result<Vec<NeighborPair>, SpatialError>

Enumerates unique unordered pairs no further apart than cutoff.

Results are sorted by (first, second) for every backend. Self-pairs are excluded, including when the two selections overlap.

Errors

Returns an error for an invalid cutoff or an out-of-range atom index.