Classical molcular dynamics
We use classical Molecular dynamics coupled to advanced sampling techniques to derive structural models of complex systems, e.g. nanoparticles and nanostructured materials, which we then refine using ab initio methods.
We studied homogeneous ice nucleation from supercooled water and ice nucleation in water droplets by combining the forward flux sampling method with molecular dynamics simulations; we used a coarse-grained water model. We found direct computational evidence that in supercooled water nano-droplets ice nucleation rates are strongly size dependent and at the nanoscale they are several orders of magnitude smaller than in bulk water.
Using forward-flux MD techniques and empirical potentials, we have shown that the presence of surfaces may help the freezing of Si and Ge, that is it may enhance nucleation rates by several orders of magnitude with respect to those found in the bulk. The main reason behind surface-induced nucleation is the way temperature varies with pressure close to the freezing point. Silicon (like germanium and water) shows a variation of temperature with pressure which is opposite to that observed in the great majority of simple materials: Si expands close to the freezing point, instead of contracting, as most materials do.
- "Surface induced crystallization in supercooled tetrahedral liquids", T. Li, D. Donadio, L. Ghiringhelli and G. Galli, Nat. Mat. 8, 726 (2009)0
- "Nucleation of tetrahedral solids: A molecular dynamics study of supercooled liquid silicon", T. Li, D.Donadio and G.Galli, J. Chem. Phys. 131, 224519 (2009)