Abstract:  Nucleation is the process that creates the first stable embryo of a new phase to initiate a phase transition.Even for simple single component systems, nucleation processes with long induction times pose serious challenges for molecular simulation.The difficulties are even more acute for multi-component condensed phase nucleation processes where most applications lie.  Recent advances are making it possible to understand the mechanisms of solute precipitate nucleation using molecular simulation.  I will use 'rare event' simulation methods test hypotheses about homogeneous methane hydrate nucleation for the first time at realistic conditions for hydrate formation.  Our results point to alternative mechanisms and suggest a potentially general and useful method to generate high driving forces for nucleation at a specific controlled location.  I will also show how computational methods can reveal the free energy landscape and mechanism that governs selection between different crystal structures, i.e. polymorphs.  The framework for understanding polymorph selection will use the crystallization of an oppositely charged colloid mixture and glycine nucleation as examples. References Peters, J. Chem. Phys. 131, (2009). Knott, Molinero, Doherty, Peters, J. Am. Chem. Soc. 134, (2012) Duff, Peters, J. Chem. Phys. 131, (2009). Duff, Peters, J. Chem. Phys. 134, (2011). Poon et al. in preparation Bio:  http://www.chemengr.ucsb.edu/people/faculty_d.php?id=43