  A novel and powerful method is presented for the study of rare switching
events in complex systems with multiscale energy landscapes. The method
performs an umbrella sampling of the equilibrium distribution of the system in
hyperplanes normal to the effective transition pathway, i.e. the transition
pathway of a coarse-grained potential which need not to be known beforehand.
Rather the effective transition pathway is determined on the fly in an adaptive
way by evolving a smooth curve, called a string, that connects the initial and
final regions and feels thermally averaged potential forces. Appropriate
averages around the string determine the transition rates. Application to an
example of solid-solid transformation of a condensed system is presented.
