While the xTB methods were parametrized mainly for molecular systems, the application to extended systems under periodic boundary conditions like molecular crystals is an area where fast semiempirical methods can be applied. A full periodic implementation of the xTB methods including k-mesh integration has been realized only in the DFTB+ package and the Atomic Simulation Environment (ASE 171 ) via the tblite library. The only other known implementation of GFN1-xTB with k-mesh in-tegration is available in the AMS package. Starting with xtb version 6.7.0, the generic force field GFN-FF supports periodic boundary conditions, enabling molecular dynamics simulations, pre-optimization, and screening of large unit cells. 173 This implementation features a specialized run mode optimized for molecular crystals (mcGFN-FF) which reduces noncovalent interactions. Despite being less robust, mcGFN-FF achieves accuracy comparable to the GFN1-xTB regarding lattice energies and unit cell volume in many cases. Exploring the initial stages of crystal structure prediction is a promising application that could greatly benefit from this force field. For non-molecular crystals, have developed the periodic GFN-FF (pGFN-FF) which utilizes the Wolf summation and damping of the bonded three-body term to stabilize electrostatic interactions, particularly for ionic crystals.