🚧 g-xTB — Development Version

This is a preliminary version of g-xTB, a general-purpose semiempirical quantum mechanical method approximating ωB97M-V/def2-TZVPPD properties for all elements up to lawrencium (Z = 1–103).

📄 Preprint

See the ChemRxiv preprint for the first g-xTB version. The present version includes substantial improvements and changes beyond the preprint.

📦 Installation

We provide statically linked binaries of the xtb program (modified version 6.7.1), interfacing with a modified version of the tblite library. Binaries are available for Linux, Windows, and ARM-based macOS.

Extract the tarball or ZIP archive for your operating system from the binaries directory into a location on your PATH (for example, $HOME/bin). No external parameter file is required.

Warning

The macOS and Windows packages include shared libraries (.dylib or .dll) that must be discoverable at runtime. In most cases, adding the bin directory to your PATH is sufficient. On macOS, external .dylib files may trigger security warnings.

Usage

The xtb binary is largely equivalent to the current bleeding-edge xtb version. To run a g-xTB calculation, simply add the --gxtb flag. For general xtb usage, see the documentation or run xtb --help.

Singlepoint calculation

xtb struc.xyz --gxtb

You can specify the total charge with --chrg or a .CHRG file (default: neutral), and the number of unpaired electrons with --uhf or a .UHF file (default: singlet for systems with an even number of electrons, doublet for systems with an odd number of electrons).

Open-shell g-xTB calculations use an unrestricted wavefunction, which is always triggered if --uhf or a .UHF file is present, even if the number of unpaired electrons is set to zero.

Control the verbosity of printed properties with --verbose and --silent.

Geometry optimization

xtb struc.xyz --gxtb --opt

Constraints, scans, molecular dynamics, and related features can be controlled through the xcontrol input file. To print the gradient, add the --grad flag.

Numerical Hessian

xtb struc.xyz --gxtb --hess

Computes a numerical Hessian based on the analytic gradient. For the numerical derivative, tight SCF convergence is beneficial. --acc defines a multiplicative factor for the convergence criteria (recommended for hessians: 0.1–0.01).

Warning

The macOS binary has a problem with the diagonalization during parallel numerical Hessian computations (serial runs are not affected).

Molden file

xtb struc.xyz --gxtb --molden

Writes a Cartesian Molden file including basis-set and orbital information for visualization and post-processing.

Solvation

g-xTB currently has only limited solvation support through either the generalized Born model with finite epsilon (--gbe) or the domain-decomposition conductor-like screening model (--cosmo). Both models include only the electrostatic contribution to the solvation free energy. A properly parameterized solvation model for g-xTB is already under development.

xtb struc.xyz --gxtb --gbe toluene
xtb struc.xyz --gxtb --cosmo water

Warning

Preliminary testing indicates that the current form of gbe can be unstable during geometry optimization. In addition, the current cosmo implementation does not include the solute response to the solvent polarization, which makes the gradient inconsistent. Both models should therefore be used with caution, and results should be evaluated carefully.

⚙️ Known limitations

At present, not all xtb features are supported for g-xTB. Known limitations include:

• the aISS docking module (dock)
• orbital localization (--lmo)
• point charge embedding ($pcem)
• cube file generation ($cube)

If you encounter another limitation or a bug, please open an issue!