This plugin adds an interface to the xtb program in Avogadro 2, as well as the CREST program for extended functionality.

xtb is developed by the Grimme group in Bonn and carries out semi-empirical quantum mechanical calculations using the group's extended Tight-Binding methods, referred to as "GFNn-xTB".

These methods provide fast and reasonably accurate calculation of Geometries, Frequencies, and Non-covalent interactions for molecular systems with up to roughly 1000 atoms, with broad coverage of the periodic table up to Z = 86 (radon).

crest (Conformer–Rotamer Ensemble Sampling Tool) adds a variety of sampling procedures for several interesting applications including conformer searches, thermochemistry, and solvation.

This plugin currently provides functionality to run the following calculation types and view the results directly in Avogadro:

• single point energies
• geometry optimizations
• vibrational frequencies
• molecular orbitals
• molecular dynamics

There is also the option to submit a custom command to xtb, permitting any calculation to be run, though the output is in this case only parsed for results of the basic calculation types.

• conformer searches

Currently, the plugin does not download the xtb binary automatically, and it is not bundled with Avogadro. Instead, it must be obtained separately. There are five options that will make xtb visible to the extension:

1. Install xtb with conda from conda-forge and use Avogadro with the conda environment, either by setting it in the Python settings or by starting Avogadro with the environment activated
2. Manually download the xtb binary and put it into the system PATH
3. Manually download xtb and place it entirely within the plugin directory <user data location>/OpenChemistry/Avogadro/commands/avo_xtb/ (see https://two.avogadro.cc/develop/scripts/install.html#directories for more information on where this is on your system)
4. Manually download xtb and manually specify its location in the Configure... menu
5. Use the "Get xtb..." function within Avogadro after installing this extension

While xtb is cross-platform, crest is distributed only for Linux/UNIX systems. As a result, Windows and macOS users of the plugin will not have the calculations that rely on CREST available to them in the Avogadro interface.

crest can be made visible to the plugin in the same ways as for xtb listed above. If method 3 is used, crest should be located at Avogadro/commands/avo_xtb/bin/crest. The "Get xtb..." option within Avogadro will also download crest on supported operating systems.

The plugin also relies on Open Babel for conversion of output files to Avogadro's cjson format. The path to the obabel binary can be specified in the Configure... menu if it is not automatically detected.

• If you use Avogadro on Windows, macOS, or the AppImage or Flatpak on Linux, you have a recent build of obabel with CJSON support in the same folder as the Avogadro executable, and you should use that. The plugin should hopefully find it automatically.
• If you have obtained it from a distro's repositories, your version of obabel does not have the necessary CJSON support, and you will unfortunately have to compile it yourself from the latest code.

xtb and crest are distributed by the Grimme group under the LGPL license v3. The authors of Avogadro and avo_xtb bear no responsibility for xtb or CREST or the contents of the respective repositories. Source code for the programs is available at the repositories linked above.

General reference to xtb and the implemented GFN methods:

• C. Bannwarth, E. Caldeweyher, S. Ehlert, A. Hansen, P. Pracht, J. Seibert, S. Spicher, S. Grimme WIREs Comput. Mol. Sci., 2020, 11, e01493. DOI: 10.1002/wcms.1493

For GFN2-xTB (default method):

• C. Bannwarth, S. Ehlert and S. Grimme., J. Chem. Theory Comput., 2019, 15, 1652-1671. DOI: 10.1021/acs.jctc.8b01176

For CREST:

• P. Pracht, S. Grimme, C. Bannwarth, F. Bohle, S. Ehlert, G. Feldmann, J. Gorges, M. Müller, T. Neudecker, C. Plett, S. Spicher, P. Steinbach, P. Wesołowski, F. Zeller, J. Chem. Phys., 2024, 160, 114110. DOI: 10.1063/5.0197592
• P. Pracht, F. Bohle, S. Grimme, Phys. Chem. Chem. Phys., 2020, 22, 7169-7192. DOI: 10.1039/C9CP06869D

See the xtb and CREST GitHub repositories for other citations.