In solid state physics and condensed matter physics, the density of states (DOS) of a system describes the proportion of states that are to be occupied by the system at each energy. The density of states is defined as
D
(
E
)
=
N
(
E
)
/
V
{\displaystyle D(E)=N(E)/V}
, where
N
(
E
)
δ
E
{\displaystyle N(E)\delta E}
is the number of states in the system of volume
V
{\displaystyle V}
whose energies lie in the range from
E
{\displaystyle E}
to
E
+
δ
E
{\displaystyle E+\delta E}
. It is mathematically represented as a distribution by a probability density function, and it is generally an average over the space and time domains of the various states occupied by the system. The density of states is directly related to the dispersion relations of the properties of the system. High DOS at a specific energy level means that many states are available for occupation.
Generally, the density of states of matter is continuous. In isolated systems however, such as atoms or molecules in the gas phase, the density distribution is discrete, like a spectral density. Local variations, most often due to distortions of the original system, are often referred to as local densities of states (LDOSs).