A note on perfect scalar fields
  We derive a condition on the Lagrangian density describing a generic, single,
non-canonical scalar field, by demanding that the intrinsic, non-adiabatic
pressure perturbation associated with the scalar field vanishes identically.
Based on the analogy with perfect fluids, we refer to such fields as perfect
scalar fields. It is common knowledge that models that depend only on the
kinetic energy of the scalar field (often referred to as pure kinetic models)
possess no non-adiabatic pressure perturbation. While we are able to construct
models that seemingly depend on the scalar field and also do not contain any
non-adiabatic pressure perturbation, we find that all such models that we
construct allow a redefinition of the field under which they reduce to pure
kinetic models. We show that, if a perfect scalar field drives inflation, then,
in such situations, the first slow roll parameter will always be a
monotonically decreasing function of time. We point out that this behavior
implies that these scalar fields can not lead to features in the inflationary,
scalar perturbation spectrum.
