# 13. Path-like objects¶

DEMETER offers a number of specialized objects that are extensions of the concept of a Path. Each of these can be treated in many ways just like a normal Path object – all of them can be plotted in the normal manner and most of them can be used as part of a fitting model. However, each encapsolates a useful concept and offers a bit of high-level functionality beyond the normal use of FEFF and IFEFFIT.

The different kinds of path-like objects are:

**VPath**- A “virtual path” is a collection of actual paths that are summed together before plotting. A VPath is only a visualisation tool and cannot be part of a fit.
**FSPath**- A “first shell path” is a tool for modeling first shell data with one single scattering path.
**FPath**- A “filtered path” is created from χ(k) data and can be used as fitting standard. This can be used to make an empirical fitting standard from measured χ(k) data. It has also been used to condense the contributions from a histogram representing structural disorder into a single path-like object.
**SSPath**- A “single scattering path” uses one scattering potential from an existing FEFF calculation to make a single scattering path at an arbitrary distance.
**MSPath**- A “multiple scattering path” uses one or more scattering potentials from an existing FEFF calculation to make a multiple scattering path from an arbitrary collection of atoms.
**ThreeBody**- A “three-body path” uses one or more scattering potentials from an existing FEFF calculation to make a double and a triple scattering path from an arrangement of three atoms.
**Forward**- A forward path is a tool for modeling the effect of changing scattering angle on a collection of three atoms in a nearly collinear arrangement. This includes the contribution for the triple scattering path and a set of double scattering paths which are used to do a interpolative approximation of the effect of changing scattering angle on the double scattering path.
**StructuralUnit**- A “structural unit” is a collection of paths, path-like objects, and GDS objects that represent a structural moiety in a fit. The idea is that a structural unit can be incorporated into a fit to represent some aspect of the structural model.

Note

MSPath, ThreeBody, Forward, and StructuralUnit objects have not yet been implemented.

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