Jacob D. Stevenson

On the surface of glasses

Dynamics near the surface of glasses is generally much faster than in the bulk. Neglecting static perturbations of structure at the surface, we use random first order transition (RFOT) theory to show the free energy barrier for activated motion near a free surface should be half that of the bulk at the same temperature. The increased mobility allows the surface layers to descend much further on the energy landscape than the bulk ordinarily does. The simplified RFOT calculation, however, predicts a limiting value for the configurational entropy a vapor deposited glass may reach as a function of deposition rate. We sketch how mode coupling effects extend the excess free surface mobility into the bulk so that the glass transition temperature is measurably perturbed at depths greater than the naive length scale of dynamic cooperativity.

A universal origin for secondary relaxations in supercooled liquids and structural glasses.

In a glassy system, a distribution of relaxation times indicates a system that continues to rearrange itself. Besides the main relaxations involved in the glass transition, there are faster dynamics associated with secondary relaxations, which are predicted to reconfigure structures that are stringy rather than tightly clustered.

Benchmarks for Characterization of Minima, Transition States, and Pathways in Atomic, Molecular, and Condensed Matter Systems.

A set of benchmark systems is defined to compare different computational approaches for characterizing local minima, transition states, and pathways in atomic, molecular, and condensed matter systems. Comparisons between several commonly used methods are presented. The strengths and weaknesses are discussed, as well as implementation details that are important for achieving good performance. All of the benchmarks and methods are provided in an online database to make the implementation details available and the results reproducible. While this paper provides a snapshot of the benchmark results, the online framework is structured to be dynamic and incorporate new methods and codes as they are developed.

Constructing explicit magnetic analogies for the dynamics of glass forming liquids

By defining a spatially varying replica overlap parameter for a supercooled liquid referenced to an ensemble of fiducial liquid state configurations, we explicitly construct a constrained replica free energy functional that maps directly onto an Ising Hamiltonian with both random fields and random interactions whose statistics depend on the liquid structure. Renormalization group results for random magnets when combined with these statistics for the Lennard-Jones glass suggest that discontinuous replica symmetry breaking would occur if a liquid with short range interactions could be equilibrated at a sufficiently low temperature where its mean field configurational entropy would vanish, even though the system strictly retains a finite configurational entropy.

Energy landscapes for a machine learning application to series data

Methods developed to explore and characterise potential energy landscapes are applied to the corresponding landscapes obtained from optimisation of a cost function in machine learning. We consider neural network predictions for the outcome of local geometry optimisation in a triatomic cluster, where four distinct local minima exist. The accuracy of the predictions is compared for fits using data from single and multiple points in the series of atomic configurations resulting from local geometry optimisation and for alternative neural networks. The machine learning solution landscapes are visualised using disconnectivity graphs, and signatures in the effective heat capacity are analysed in terms of distributions of local minima and their properties.

Structural analysis of high-dimensional basins of attraction

We propose an efficient Monte Carlo method for the computation of the volumes of high-dimensional bodies with arbitrary shape. We start with a region of known volume within the interior of the manifold and then use the multistate Bennett acceptance-ratio method to compute the dimensionless free-energy difference between a series of equilibrium simulations performed within this object. The method produces results that are in excellent agreement with thermodynamic integration, as well as a direct estimate of the associated statistical uncertainties. The histogram method also allows us to directly obtain an estimate of the interior radial probability density profile, thus yielding useful insight into the structural properties of such a high-dimensional body. We illustrate the method by analyzing the effect of structural disorder on the basins of attraction of mechanically stable packings of soft repulsive spheres.

Exploiting the potential energy landscape to sample free energy

We review a number of recently developed strategies for enhanced sampling of complex systems based on knowledge of the potential energy landscape. We describe four approaches, replica exchange, Kirkwood sampling, superposition‐enhanced nested sampling, and basin sampling, and show how each of them can exploit information for low‐lying potential energy minima obtained using basin‐hopping global optimization. Characterizing these minima is generally much faster than equilibrium thermodynamic sampling, because large steps in configuration space between local minima can be used without concern for maintaining detailed balance. WIREs Comput Mol Sci 2015, 5:273–289. doi: 10.1002/wcms.1217

Research data supporting [Defining and Quantifying frustration in the energy landscape: Applications to atomic and molecular clusters, biomolecules, jammed and glassy systems].

Frustration in the energy landscape has been quantified using a number of different metrics. These metrics have been applied to the databases in the subdirectories, which contain information about the energy landscape in the form of minima and transition states. The frustration metrics are coded within The PATHSAMPLE program, available for download at: http://www-wales.ch.cam.ac.uk/PATHSAMPLE/ Documentation can also be found at the same web address. The relevant keywords for the work described in this paper can be found at: http://www-wales.ch.cam.ac.uk/PATHSAMPLE.2.1.doc/node5.html Keywords CV, SHANNON and SHANNONR are described in the documentation. SHANNON and SHANNONR calculate frustration measures for the landscape as a function of temperature. The CV keyword gives a heat capacity calculation and hence allows an estimate of the melting temperature. Each Directory holds the database for the relevant system and input files for the PATHSAMPLE program. The output of the frustration calculation, for SHANNON, is given in Shannon.out. Some databases also contain output for the heat capacity calculation in CV.out. If required the OPTIM program is also available for download at: http://www-wales.ch.cam.ac.uk/OPTIM/ Documentation can also be found at the same web address. The program for drawing disconnectivity graphs, disconnectionDPS, can be found at: http://www-wales.ch.cam.ac.uk/software.html There is also input and output for the disconnectivity graph construction in each directory.