🚧 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.

📄 Preprint

See the preprint at ChemRxiv: https://chemrxiv.org/engage/chemrxiv/article-details/685434533ba0887c335fc974

📦 Installation

Warning

gxtb currently works only on Linux-based machines.

Place the gxtb binary in a directory belonging to your $PATH variable (e.g., $USER/bin/).

Place the following parameter and basis files into a dedicated directory, which you export in the $GXTBHOME variable:

• .gxtb — parameter file (-p)
• .eeq — electronegativity equilibration parameters (-e)
• .basisq — atom-in-molecule AO basis (-b) If $GXTBHOME is not defined, the gxtb binary searches first in your home directory $HOME and then in the current directory (./). You can overwrite the location of the parameter files with the resepctive command-line flags (-p, -e, and -b).

Usage

By default, gxtb expects a coordinate file in TURBOMOLE format (coord) using atomic units (i.e. Bohr).

Run examples

gxtb                       # default: coord file = TURBOMOLE format
gxtb -c <coord_file_name>  # explicit coordinate file (TURBOMOLE or XYZ)
gxtb -c <xyz_file_name>    # XYZ file supported

Place the following optional control files in your working directory:

• .CHRG # Integer charge of the system (default: neutral)
• .UHF # Integer number of open shells (e.g., 2 for triplet, 0 for singlet UKS)

If .CHRG or .UHF are not present:

• Even electrons: neutral singlet (RKS)
• Odd electrons: neutral doublet (UKS)

⚙️ Additional Features

Numerical Gradient

gxtb -grad

Or if a file named .GRAD is present, a numerical gradient is computed (expensive!). Molecular symmetry is exploited to speed up calculations.

Geometry Optimization with xtb

To optimize geometries using xtb with gxtb as a driver:

xtb struc.xyz --driver "gxtb -grad -c xtbdriver.xyz" --opt

Or with a coord file in TURBOMOLE format:

xtb coord --driver "gxtb -grad -c xtbdriver.coord" --opt

💡 You may use --opt loose for faster convergence, as there is currently no analytical nuclear gradient — gradients are evaluated numerically and can be noisy.

Numerical Hessian

gxtb -hess

Computes a numerical Hessian (very expensive).

Current Coverage

• Reasonably parameterized for elements Z = 1–58, 71–89, and 92
• A revised dispersion model (revD4) is in progress and may slightly affect final results

📊 Output and Analysis

• All computed properties aim to approximate ωB97M-V/def2-TZVPPD
• EEQ_BC charges mimic Hirshfeld charges from that reference
• Use the -molden flag to write a .molden file with orbitals and basis info:

gxtb -molden

Useful for visualization and post-processing.