  Following a recent proposal, we delineate a general procedure to classify 5d
SCFTs via compactifications of 6d SCFTs on a circle (possibly with a twist by a
discrete global symmetry). The path from 6d SCFTs to 5d SCFTs can be divided
into two steps. The first step involves computing the Coulomb branch data of
the 5d KK theory obtained by compactifying a 6d SCFT on a circle of finite
radius. The second step involves computing the limit of the KK theory when the
inverse radius along with some other mass parameters is sent to infinity. Under
this RG flow, the KK theory reduces to a 5d SCFT. We illustrate these ideas in
the case of untwisted compactifications of rank one 6d SCFTs that can be
constructed in F-theory without frozen singularities. The data of the
corresponding KK theory can be packaged in the geometry of a Calabi-Yau
threefold that we explicitly compute for every case. The RG flows correspond to
flopping a collection of curves in the threefold and we formulate a concrete
set of criteria which can be used to determine which collection of curves can
induce the relevant RG flows, and, in principle, to determine the Calabi-Yau
geometries describing the endpoints of these flows. We also comment on how to
generalize our results to arbitrary rank.
