**2. Hydroxyapatite and Bioglass® as computational case study**

All the calculations mentioned in this Chapter have been performed using the CRYSTAL code in its latest release (Dovesi et al., 2005a; Dovesi et al., 2005b; Dovesi et al., 2009). This periodic quantum-mechanical software has been developed by the Theoretical Chemistry group of the University of Turin (Italy) together with the Daresbury Laboratory (UK) since 1988. CRYSTAL is capable of computing systems with every dimensionality, from molecules to real infinite crystals and it supports massive parallel calculations. This code uses local Gaussian basis sets and can deal with many electronic structure methods, from Hartree-Fock to Kohn-Sham Hamiltonians. Structural, electrostatic and vibrational properties of the studied materials have been characterized with the program. Another crucial aspect in modeling is the graphical visualization and representation of structures. For all the images displayed in this Chapter, MOLDRAW (Ugliengo et al., 1993), J-ICE (Canepa et al., 2011b) and VMD (Humphrey et al., 1996) programs were used. Further more precise computational details can be read in a number of our recent papers on both HA (Corno et al., 2009; Corno et al., 2006; Corno et al., 2007; Corno et al., 2010) and bioactive glasses (Corno & Pedone, 2009; Corno et al., 2008).
