Tracing the Entropy along a Reactive Pathway: The Energy As a Generalized Reaction Coordinate.

By using metadynamics at a temperature T0 we reconstruct the free energy FT0(E,s) as a function of the potential energy E and of a geometrical variable s. We show here that from FT0(E,s) one can estimate the free energy also at a different temperature. This allows tracing the entropy and characterizing the properties of molecular systems at all temperatures by a single simulation. We validate this approach on the water dimer dissociation.