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A
physical system
is said to be in a particular state when its physical properties fall within
some particular range; the boundaries of the range defining a state depend on
the problem under consideration. In a classical world, each point in phase space could be said to correspond to a distinct
state. In the real world, time-invariant systems
in quantum
mechanics have a set of discrete states, particular superpositions of which constitute complete
descriptions of the system.
In practice, broader boundaries are usually drawn. A molecule is often said to be in a particular
excited electronic
state, regardless of its state of mechanical
vibration. In nanomechanical systems,
the PES often corresponds to a set of distinct potential wells, and all points in configuration space within a
particular well can be regarded as one state. Definitions of state in the thermodynamics of bulk matter are analogous,
but extremely coarse by these standards.
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