axtb

Extended tight binding (GFN2-xTB; g-xTB v1.1 and 2.0.1) and the GFN-FF force field, in-process from Python: energies, analytic gradients, Hessians with vibrational analysis and thermochemistry, implicit solvation (ALPB/GBSA for GFN2-xTB, and for g-xTB through a composite), Molden files, bond orders. Molecules only. The engine is axtb, written in Rust.

In a reference program's conventions axtb reproduces that program's numbers to the last digits (xtb 6.8.0 and 6.7.1, tblite 0.8.0, the g-xTB 1.1.0 and 2.0.1 programs); its default conventions use CODATA 2022 constants and correct the program bugs found on the way. Calculations run on the threads you pass (default 1) and never spawn more, so many can run side by side.

import axtb
pos = [[0, 0, 0.22], [0, 1.43, -0.89], [0, -1.43, -0.89]]          # bohr, any array-like
r = axtb.singlepoint([8, 1, 1], pos, method="gfn2", gradient=True)
r["energy"], r["gradient"]
calc = axtb.Calculator("gxtb-v2.0.1", [8, 1, 1], pos, threads=4)   # reuses setup and wavefunction
h = calc.hessian(pos)                                              # analytic Hessian and its analysis
h["frequencies"], h["ir_intensities"], h["thermo"]["free_energy"]

With pip install "axtb[qc]" the package also registers a codec for qccodec, so qccompute runs axtb like any other program.

Requirements. The wheels are for x86-64 Linux with glibc 2.28 or newer and Python 3.10 or newer, and need an x86-64-v3 CPU (AVX2, FMA: Intel since Haswell, AMD since Excavator). import axtb checks the CPU and raises ImportError on one without these instructions. On other platforms pip builds the package from the source distribution, which needs Rust 1.99 or newer.

Documentation. The README describes the methods, conventions and settings; docs/validation.md has the agreement with each program; the changelog lists what each release contains.

License. GPL-3.0-only (COPYING), because the package includes g-xTB, whose parameters are GPL-3.0 (grimme-lab/g-xtb). The rest of axtb is LGPL-3.0-or-later (COPYING.LESSER), and a build without g-xTB (maturin build --no-default-features) is LGPL-3.0-or-later. LICENSE explains the split; NOTICE credits the upstream work (xtb, tblite, dftd4, mctc-lib, multicharge, g-xTB).