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Kwant 2.x will allow for more general systems to be considered than Kwant 1.x.
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[[Previously | Low-level-System-Format]] a proposal was drawn up to extend the
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fundamental low-level system format. Such structures are characterised by a
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symmetry group and different "terms" of the Hamiltonian are associated with the
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different symmetry group elements. Such structures do not, however, enable one
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to treat problems such as a central scattering region attached to semi-infinite,
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periodic leads. This page describes a proposal for "Composite" systems, that
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will enable treatment of such systems.
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## Requirements
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Such a composite system should enable the treatment of general scattering
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problems in arbitrary dimensions. Examples include:
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+ Finite scattering region attached to 1D leads
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+ 1D scattering region attached to 2D leads (+ higher dimensional
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generalizations)
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+ Finite scattering region embedded in 2D bulk (+ higher dimensional
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generalizations)
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**TODO**: put examples w/ illustrations here
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## `CompositeSystem` API
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A `CompositeSystem` is essentially a collection of `System`s arranged in a
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directed graph, where systems of higher symmetry are *attached* to systems
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of a lower symmetry. The interface between two systems is specified by a
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sequence of *terms* of the system with higher symmetry that join the
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fundamental domain of the system with lower symmetry to the system
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with higher symmetry. If all the terms associated with a particular
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group element (of the higher-symmetry system) are present in the
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interface, we say that the lower-symmetry system *fully interrupts*
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the higher-symmetry one.
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The system with lower symmetry must contain a full fundamental domain
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of the system with higher symmetry, otherwise the `term` does not
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represent a well defined hopping into the fundamental domain of the
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lower symmetry.
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The following examples should serve to illustrate the point.
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**TODO**: put examples w/ illustrations here |