In much the same way that the potential energy surface of a molecule dictates the motion of its atoms, the potential energy landscape of a liquid acts to define the dynamics of its constituent molecules. What is remarkable about liquids, though, is how much of that dynamics seems to be controlled by basic features of the landscape that have nothing to do with where the minima and maxima are. This talk looks at two different takes on those fundamental landscape features. One of these, the so-called instantaneous normal modes of liquids, offers a way of interpreting much of what is revealed by ultrafast spectroscopy. The other, the geodesic (shortest) paths through the potential surface, suggests how we might want to think about much slower events in liquids -- such as the extraordinarily slow diffusion characteristic of supercooled liquids. In both cases, it is the ability to consider the cooperative motion of whole molecular assemblages that ends up removing some of mystery behind the apparently erratic dynamics displayed by individual molecules in liquids.

Two steps forward and (not quite) two steps back? What the potential energy landscape of a liquid tells you about the geometry of molecular motion in liquids

ACS Spring 2022

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