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International audience; The Crystal program for quantum-mechanical simulations of materials has been bridging the realm of molecular quantum chemistry to the realm of solid state physics for many years, since its first public version released back in 1988. This peculiarity stems from the use of atom-centered basis functions within a linear combination of atomic orbitals (LCAO) approach and from the corresponding efficiency in the evaluation of the exact Fock exchange series. In particular, this has led to the implementation of a rich variety of hybrid density functional approximations since 1998. Nowadays, it is acknowledged by a broad community of solid state chemists and physicists that the inclusion of a fraction of Fock exchange in the exchange-correlation potential of the density functional theory is key to a better description of many properties of materials (electronic, magnetic, mechanical, spintronic, lattice-dynamical, etc.). Here, the main developments made to the program in the last five years (i.e., since the previous release, Crystal17) are presented and some of their most noteworthy applications reviewed.

We report a series of computations on the active site in Ti-substituted zeolites, specifically TS-1. Hybrid QM/MM methods based on density functional calculations using the BB1K functional and a valence force field are used to study the processes of hydrolysis of Ti–O–Si linkages and inversion of the TiO4 tetrahedra. The structural features of the resulting series of tetra- and tripodal Ti moieties are in good agreement with data from EXAFS studies. The suggestion is made that the tripodal species will dominate in hydrous conditions, and that this is likely to be the chemically active form. We have made extensive use of the massively parallel HPCx computer system for these investigations and outline some of the technical developments to the ChemShell software that were needed to support the study.