Semiempirical Tight-Binding

xTB

Use GFN methods for low-cost structure relaxation, conformer screening, frequencies, and approximate electronic properties before higher-level calculations.

Role: Semiempirical tight-binding
Typical input: XYZ, charge, spin, solvent
Typical output: Energies, structures, Hessians

1. Scope of xTB

GFN-xTB methods are parameterized for geometries, frequencies, and noncovalent interactions at much lower cost than routine DFT. They are valuable for screening, but method-dependent errors and electronic-state limitations remain.

Check the domain of applicability

Benchmark charge states, open-shell species, transition metals, unusual bonding, reaction barriers, and small selectivity differences against an appropriate higher-level method or experiment.

2. GFN methods

Method	Typical use
GFN2-xTB	General molecular structures and noncovalent interactions
GFN1-xTB	Fast geometry and vibrational work
GFN-FF	Very large-scale conformational screening

GFN0-xTB is an additional fast electronic method. GFN-FF is a force field rather than an electronic-structure method, so electronic properties available from GFN-xTB must not be assumed to exist for GFN-FF.

3. Single points and optimization

Shell

xtb structure.xyz --gfn 2 --chrg 0 --uhf 0
xtb structure.xyz --opt tight --gfn 2
xtb xtbopt.xyz --hess --gfn 2

--chrg is the total charge and --uhf is the number of unpaired electrons, not the spin multiplicity. Inspect the output for SCC convergence and the final total energy.

4. Geometry optimization

xtb structure.xyz --opt tight --gfn 2 --chrg 0 --uhf 0
# Optimized coordinates are normally written to xtbopt.xyz.

Optimization convergence does not establish whether the structure is a minimum. Inspect chemical connectivity and follow with a Hessian when the stationary-point character matters.

5. Hessian, frequencies, and thermochemistry

xtb xtbopt.xyz --hess --gfn 2 --chrg 0 --uhf 0

The Hessian provides approximate vibrational frequencies and thermal corrections. Imaginary modes indicate that the geometry is not a minimum, subject to numerical noise and the treatment of very soft modes.

6. Solvent models

GBSA and ALPB provide implicit-solvent corrections. Record the solvent model and solvent name, and do not mix gas-phase and solvated energies without a defined thermodynamic cycle.

Shell

xtb structure.xyz --opt --alpb chloroform

Implicit solvent does not represent specific solvent molecules or every change in association equilibrium. Include explicit solvent molecules when a mechanistically essential interaction requires them, and state how they were sampled.

7. Screening workflow

Generate diverse conformers.
Optimize consistently with fixed charge and spin.
Remove duplicates using geometry and energy.
Refine several candidates with DFT.
Compare whether relative ordering is stable.

For thermochemistry, confirm that the optimized geometry is a minimum and inspect low-frequency modes. Store the xTB version, method, charge, UHF value, solvent model, optimization level, and command line with every result.

8. References

Last reviewed: August 4, 2026. Check the linked official documentation for syntax specific to the installed software version.