xTB
A semiempirical extended-tight-binding program implementing GFN-family methods for inexpensive geometries, frequencies and molecular screening workflows.
Where it fits
Rapid preoptimization and screening before more expensive electronic-structure calculations.
Limits and practical considerations
Semiempirical errors vary with chemistry; important energies and unusual bonding require higher-level validation.
Licensing and access
LGPL version 3 or later; related tools are separate projects with their own terms.