(536b) On Methods for Computing Free Energy Differences and Barriers in Soft Materials Undergoing Ordering Transitions

Accurate computation of free energies plays a key role in molecular simulations aimed to elucidate the thermodynamic or kinetic behavior of complex systems involving oligomers, polymers or nanoparticles. This is particularly important in understanding disorder-to-order transitions in bulk materials and to discriminate among stable and metastable states. Ordered phases of interest include crystalline assemblies formed by nanoparticles and mesophases formed by the microphase segregation of polyphilic, multiblock molecules.

After reviewing some of the general statistical mechanical principles involved in modeling free energies and how the connection among some old and new methodological variants are often overlooked (an issue that generalized frameworks try to remedy), I will describe selected methods that our group has developed to efficiently evaluate free energy changes associated with different phases and with the transition barriers between phases. These include methods for optimizing multistage ensemble simulations, for tracing phase boundaries, and for estimating free-energy nucleation barriers for situations posing significant challenges to conventional methods. I will illustrate the usefulness of these methods through sample applications concerning the self-assembly behavior of novel particle-based crystals and oligomeric mesophases. I will conclude by pointing out some of the outstanding methodological challenges, e.g., associated with mapping and inverse-mapping the behavior of coarse and fine-grained models.