Short- and medium-range structure of multicomponent bioactive glasses and melts: An assessment of the performances of shell-model and rigid-ion potentials

Classical and ab initio molecular dynamics (MD) simulations have been carried out to investigate the effect of a different treatment of interatomic forces in modeling the structural properties of multicomponent glasses and melts. The simulated system is a soda-lime phosphosilicate composition with bioactive properties. Because the bioactivity of these materials depends on their medium-range structural features, such as the network connectivity and the Q(n) distribution (where Q(n) is a tetrahedral species bonded to n bridging oxygens) of silicon and phosphorus network formers, it is essential to assess whether, and up to what extent, classical potentials can reproduce these properties. The results indicate that the inclusion of the oxide ion polarization through a shell-model (SM) approach provides a more accurate representation of the medium-range structure compared to rigid-ion (RI) potentials. Insight into the causes of these improvements has been obtained by comparing the melt-and-quench transformation of a small sample of the same system, modeled using Car-Parrinello MD (CPMD), to the classical MD runs with SM and RI potentials. Both classical potentials show some limitations in reproducing the highly distorted structure of the melt denoted by the CPMD runs; however, the inclusion of polarization in the SM potential results in a better and qualitatively correct dynamical balance between the interconversion of Q(n) species during the cooling of the melt. This effect seems to reflect the slower decay of the fraction of structural defects during the cooling with the SM potential. Because these transient defects have a central role in mediating the Q(n) transformations, as previously proposed and confirmed by the current simulations, their presence in the melt is essential to produce an accurate final distribution of Q(n) species in the glass.