xTB

xtb is an open-source computational chemistry software package that implements extended tight-binding methods for fast quantum-chemical calculations. It was developed to provide a computationally efficient alternative to density functional theory for large molecules and materials, offering reasonable accuracy for geometry optimization, vibrational frequencies, and noncovalent interactions.

xtb's core methods are the GFN-xTB family, including GFN1-xTB and GFN2-xTB. These are semi-empirical extended tight-binding Hamiltonians parameterized to reproduce geometries, frequencies, and noncovalent interactions across a broad range of elements. The package includes built-in dispersion corrections (notably D4) and implicit solvent models to capture solvation effects in a variety of environments. It also provides tools for quick conformer searches, transition-state guess generation, reaction paths, and approximate thermochemistry.

In addition, xtb offers approximate excited-state capabilities through simple time-dependent schemes (sTDA) suitable for screening UV-Vis properties of large systems.

xtb is cross-platform and operated from the command line, making it easy to integrate into high-throughput workflows and scripting environments. It is widely used in organic and inorganic chemistry, materials science, and biochemistry for rapid screening, preliminary geometry optimizations, and teaching purposes. Documentation and tutorials are available from the project repository and user community.