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The subsystem functional scheme is a recently proposed framework for
constructing exchange-correlation density functionals for use in
density-functional theory based calculations. The fundamental principle
is to describe the physics in a real material by mapping onto model
systems that exhibit the characteristic physics in each separate part of
the real system. The LDA functional can be seen as a subsystem
functional: in all parts of the real material the assumption is that the
needed physics is well described by the uniform electron gas model
system. It is well known that this assumption is very accurate for
surprisingly large classes of materials. The Armiento-Mattsson 2005
(AM05) functional takes this a step further by distinguishing between
two separate types of regions in a real material, one type that is
assumed to be well described by the uniform electron gas, and the other
type of region assumed to be well described by a surface model system.
AM05 gives a consistent improvement over LDA. One important consequence
of the subsystem functional scheme is that it is known what physics is
included in a functional. Based on the performance of AM05 for a number
of different systems we discuss where the model systems included are
enough and when additional physics need to be included in a new
functional. Improvement of AM05 is possible by fine-tuning the details
in the construction. But a new major step in accuracy improvement is
only expected if new physics is integrated in a functional via an
additional model system. We discuss what type of physics would be needed
and what model systems could be used for this next step beyond AM05.
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