Methods

The plug-in exposes four Hamiltonians / force fields from the xTB family. The xTB method parameter is shared by all sub-steps.

Choosing among the GFN family

GFN2-xTB (default)

The recommended general-purpose method. Self-consistent with multipole electrostatics (up to quadrupoles on each atom) and a density-dependent dispersion correction (D4 family). Parametrized for Z = 1-86.

GFN2-xTB is the right starting point for almost everything: organic, inorganic, organometallic, and main-group systems. It is the most accurate of the GFN methods for structures, conformer energies, and non-covalent interactions, and is well-tested across a broad range of benchmark sets.

Citation: Bannwarth, Ehlert, Grimme, J. Chem. Theory Comput. 2019, 15, 1652.

GFN1-xTB

The earlier self-consistent xTB Hamiltonian. Uses only monopole-monopole electrostatics and a less elaborate dispersion correction. Parametrized for Z = 1-86.

GFN1-xTB is typically less accurate than GFN2 for thermochemistry but is sometimes more robust for difficult electronic-structure cases (heavily strained systems, unusual oxidation states). Useful when GFN2 fails to converge or gives implausible geometries.

Citation: Grimme, Bannwarth, Shushkov, J. Chem. Theory Comput. 2017, 13, 1989.

GFN0-xTB

A non-self-consistent (single-shot) method. Faster than GFN1/GFN2, and very robust as a starting-point method or for very large systems. Parametrized for Z = 1-86.

Best uses: rough screening of large libraries; getting a reasonable starting geometry for a higher-level method; cases where SCC convergence is unreliable.

Note that GFN0-xTB is not parametrized for ALPB or GBSA implicit solvation. If you need implicit solvation with GFN0, use CPCM-X. The plug-in will pass the combination to xtb regardless; xtb itself will emit a warning or refuse to run.

Citation: Pracht, Caldeweyher, Ehlert, Grimme, ChemRxiv 2019, DOI 10.26434/chemrxiv.8326202.v1. (As of the plug-in v1 release this paper has only a preprint; a peer-reviewed version may appear later.)

GFN-FF

A generic, non-quantum force field automatically parametrized by xTB from the structure. No SCC, no Hessian build-up of polarization response.

GFN-FF is by far the fastest of the four and is intended for very large systems (thousands of atoms) where even GFN0 is too slow, and for MD or conformational sampling where a force field is sufficient. Accuracy is markedly lower than the SCC GFN methods – do not use GFN-FF for energy differences where you care about precision under a kcal/mol.

Citation: Spicher, Grimme, Angew. Chem. Int. Ed. 2020, 59, 15665.

Quick comparison

Method	Type	Speed	Best for
GFN-FF	Force field	Fastest	Very large systems, MD
GFN0-xTB	Non-SCC	Fast	Screening, starting point
GFN1-xTB	SCC	Moderate	Backup when GFN2 struggles
GFN2-xTB	SCC + D4	Moderate	General-purpose default

Accuracy parameter

The Accuracy field maps to xTB’s --acc flag. Lower values tighten the integral cutoffs and SCC convergence thresholds; higher values loosen them. The default of 1.0 is appropriate for almost all calculations. A typical usable range is roughly 0.0001 (very tight) to 1000 (very loose, only for crude screening).

You generally do not need to change this. The main case where tightening it helps is when you are about to do a Frequencies calculation and want a tightly converged starting geometry; the tight/vtight/extreme optimization levels already arrange for this.