A Computational Multiscale Approach to the Modeling of dynamical properties of 45S5 Bioglass®

The Hench Bioglass® 45S5 of 46.1% SiO2, 26.9% CaO, 24.4% Na2O and 2.6% P2O5 molar composition is of great interest in medical applications since in the presence of body fluids, and depending upon the rate of ion release and resorption, it creates chemical gradients which promote the formation of a layer of biologically active bone-like hydroxyapatite at the implantation interface. Osteoblasts can preferentially proliferate on the apatite layer, and differentiate to form new bone that binds strongly to the implant surface. Its simulation has been undertaken in a multiscale approach in which molecular dynamics simulations based on classical force fields have been carried out on a unit cell containing 78 atoms with Na12Ca7P2Si13O44 composition and P1 symmetry. Molecular mechanics minimization was then run on a representative quenched structure, to relax fully the system which is subsequently passed to the CRYSTAL061 code as a starting structure to perform a full ab-initio periodic geometry relaxation using the hybrid B3LYP functional with Gaussian basis sets of double-z quality. The hybrid B3LYP has been chosen because it has recently proved to be very successful in the treatment of complex silicate crystals. The full IR spectrum of the Bioglass® has been computed ab-initio and compared to experimental results and to the IR spectrum of an amorphous silica model, optimised at the same theoretical level. In spite of great complexity due to the large number of active modes, a good agreement with experimental data was found, confirming the reliability of the present model. Spectroscopic data were compared also with simulated vibrational spectrum of pure amorphous silica, to highlight the role of phosphorous and network modifiers cations. The present attempt is the starting point of a more general multiscale approach in which molecular mechanics calculations will provide initial structures for ab-initio simulation. This latter, in turn, will provide electronic features of complex materials and can also be used to refine, in a fully self consistent way, the force field parameters derived empirically as was recently done for the hydroxyapatite crystal. Present DFT functionals (hybrid ones in particular) have been proved excellent tools for the simulation of static and dynamic properties of silica-based materials. 1. Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; Zicovich-Wilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D'Arco, P.; Llunell, M. CRYSTAL2006 User's Manual. Torino, University of Turin, 2006 http://www.crystal.unito.it.