  The Refl1d program is used for modeling and fitting data from neutron and
X-ray reflectometry experiments. The model of the (thin-film) samples is
typically constructed of discrete layers of different scattering-length
densities (SLD). Interlayer roughness is represented as an error-function
transition from one layer to the next. Previous versions of the software
truncated this error-function at the next interface. This strategy has the
advantage of preventing layers with unbounded effective extent, but it can also
result in SLD depth profiles that do not conform to the physical expectations
of the users (such as introducing sharp transitions) whenever the layer
roughness approaches the thickness of the layer. In this article we introduce a
new version of the software in which the option is provided to extend the
roughness of each layer over the entire structure; the resulting SLD profiles
often more closely resemble the physical models intended by the user. Most
importantly no sharp transitions are introduced by truncating the roughness,
when a smooth transition is often desired when adding rough layers together.
