  Despite major advances in the understanding of the formation and dynamics of
nano-clusters in the past decades, theoretical bases for the control of their
shape are still lacking. We investigate strategies for driving fluctuating
few-particle clusters to an arbitrary target shape in minimum time with or
without an external field. This question is recast into a first passage
problem, solved numerically, and discussed within a high temperature expansion.
Without field, large-enough low-energy target shapes exhibit an optimal
temperature at which they are reached in minimum time. We then compute the
optimal way to set an external field to minimize the time to reach the target,
leading to a gain of time that grows when increasing cluster size or decreasing
temperature. This gain can shift the optimal temperature or even create one.
Our results could apply to clusters of atoms at equilibrium, and colloidal or
nanoparticle clusters under thermo- or electrophoresis.
