  We explore chaos in the Kuramoto model with multimodal distributions of the
natural frequencies of oscillators and provide a comprehensive description
under what conditions chaos occurs. For a natural frequency distribution with
$M$ peaks it is typical that there is a range of coupling strengths such that
oscillators belonging to each peak form a synchronized cluster, but the
clusters do not globally synchronize. We use collective coordinates to describe
the inter- and intra-cluster dynamics, which reduces the Kuramoto model to
$2M-1$ degrees of freedom. We show that under some assumptions, there is a
time-scale splitting between the slow intracluster dynamics and fast
intercluster dynamics, which reduces the collective coordinate model to an
$M-1$ degree of freedom rescaled Kuramoto model. Therefore, four or more
clusters are required to yield the three degrees of freedom necessary for
chaos. However, the time-scale splitting breaks down if a cluster
intermittently desynchronizes. We show that this intermittent desynchronization
provides a mechanism for chaos for trimodal natural frequency distributions. In
addition, we use collective coordinates to show analytically that chaos cannot
occur for bimodal frequency distributions, even if they are asymmetric and if
intermittent desynchronization occurs.
