# User Guide to Semiempirical Tight Binding¶

This user guide focuses on the semiempirical quantum mechanical methods GFNn-xTB, their descendants, and corresponding composite schemes as implemented in the xtb (extended tight binding) program package.

We provide a number of detailed guides dealing with common task that can
be performed easily with the `xtb`

program.
All guides are usually structured the same way, starting with some simple
examples using only the commandline and the default settings followed
by a trouble shooting section.
Detailed inputs are provided in a ready to use fashion to solve some
more special but still common tasks with `xtb`

together with some
insights into the theory used behind the scences.

## Quick Links¶

## Recent developments, news and publications¶

2022-05-15: xtb version 6.5.0 released by

*Sebastian Ehlert*We are happy to release a new version of

`xtb`

with exciting new features. First of all, we improved the user-friendliness of the error messages in the geometry reader by adopting our IO-library, which is already in wide use in dftd4, gcp and other projects. No more*invalid input provided*obscure error messages, but actual pointers on what went wrong with the input.We also improved the capability of existing geometry readers to keep up to date with the parent programs, for example we now support the

`$eht charge=0 unpaired=0`

line to set the system charge and number of unpaired electrons, which was added in a Turbomole 7.5. Furthermore, we are happy to have now support for QChem molecule files (`.qchem`

), FHI-aims geometry inputs (`geometry.in`

) and QCSchema formatted JSON (`.json`

).2021-12-13: DFTB+ version 21.2 released by

*Sebastian Ehlert*A new version of DFTB+ is now available with support for the xTB methods. Due to the integration in DFTB+ most of the features available for the DFTB Hamiltonian can be readily used with the xTB Hamiltonians as well, including periodic calculations with k-point sampling, geometry optimizations, molecular dynamics and frequency calculations. Both GFN1-xTB and GFN2-xTB are available for the DFTB+ version 21.2 at the moment.

Furthermore, we integrated parts the battle-proven rational function optimizer from

`xtb --opt`

into DFTB+ to allow fast and robust geometry optimizations both for molecular and periodic systems. Preliminary tests show an order of magnitude improvements in the convergence compared with the previous default (c.f. dftbplus#862).2021-08-27: QCxMS version 5.1.2 released by

*Jeroen Koopman*This new update of the QCxMS program is linked against the AVX2 processor extension for increased performance. CID and EI runmode changes and bugfixes are implemented. The local run-script pqcxms was re-written and now performs better for parallel calculations.

For a detailed description of all changes, check out the GitHub repository.

2021-08-06: QCxMS version 5.1.0 released by

*Jeroen Koopman*This new update of the QCxMS program changes the way xtb is used inside the code. Instead of a standalone implementation of xtb version 5.8.1 in the source code, we switched to using the tblite library, which allows updates to the latest version of xtb. This, in return, leads to a significant

**increase in the computational speed**of calculations done with the*GFNn-xTB*methods, while keeping the code independent from third party software.Furthermore, the PlotMS program has been updated as well.

2021-06-10: xtb version 6.4.1 released by

*Sebastian Ehlert*We released a new version of

`xtb`

with a significantly improved memory footprint for large scale calculations and improved parallelisation for frequency calculations. The parallel evaluation of hessians with GFN-FF is now possible, overall we improved the stablility of the parallelisation which was slightly degraded in version 6.4.0. For xTB calculations the required`OMP_STACKSIZE`

has been significantly reduced by restructuring the integral evaluation slightly.Also, this version of

`xtb`

now supports the COSMO/CPCM solvation model using the ddPCM library.

See the news archive for all posts.

## xTB in Other Quantum Chemistry Programs¶

The xTB-methods are now officially available in other quantum chemistry programs!

in Orca 4.2 an IO-based interface to the

`xtb`

binary is availableAMS 2019 implements GFN1-xTB in their DFTB module

the entos program implements GFN1-xTB (also available in the webinterface)

the computational chemistry framework cuby4 supports

`xtb`

Turbomole does support GFN1-xTB and GFN2-xTB since version 7.4

QCEngine supports calculations with the

`xtb`

APIthe GMIN, OPTIM, and PATHSAMPLE global optimization tools provide a

`xtb`

interfaceCP2K has an GFN1-xTB implementation since version 7.1

DFTB+ support GFN1-xTB and GFN2-xTB since version 21.2

We missed your project here? No problem, just give us hint at the mailing list or open an issue at github.

- Singlepoint Calculations
- Properties
- Geometry Optimization
- Exploration of the potential energy surface (PES)
- Implicit Solvation
- Calculation of Vibrational Frequencies
- Molecular Dyamics Simulations
- Meta-Dynamics Simulations
- Reaction Path Methods
- Growing String Method
- Periodic Boundary Conditions
- External Potentials and Embedding
- GFN-Force-Field (GFN-FF)
- C API to the extended tight binding program
- Python Integration for the xtb API
- Community resources

- Introduction to CREST
- CREST Versions and Changelog
- CREST command line arguments
- Example applications
- iMTD-GC conformational search
- Sorting an ensemble
- Comparing two ensembles
- Constrained conformational sampling
- Sampling of noncovalent complexes and aggregates (NCI mode)
- Molecular prototropy screening
- Property calculations on final ensemble
- Dry run to check settings prior to calculations
- Examples from the paper: Automated exploration of the low-energy chemical space with fast quantum chemical methods
- Preparing a Nanoreactor calculation for
`xtb`

- Quantum Cluster Growth

- Introduction to CENSO
- Setting up CENSO
- Censorc keyword definitions
- Thresholds
- Solvation
- NMR
- Trouble shooting
- Usage examples
- Calculate fast DFT(B97-D3(0)/def2-SV(P)+gcp) single-point energies on GFN
*n*-xTB input geometries - Calculate free energies in solution phase (CHCl
_{3}) on GFN*n*-xTB geometries - Calculate free energies on populated, DFT optimized conformers
- Calculation of NMR spectra
- Calculation of optical rotation
- Restarting calculations

- Calculate fast DFT(B97-D3(0)/def2-SV(P)+gcp) single-point energies on GFN
- Abbreviations