Density of states.

        :param e: Energy :math:`E`.
        :type e: float

        :return: :math:`ρ(E) = \\left|k'(E) k(E)\\right|`
        :rtype: float

def ρ(self, e):
        """Density of states.

        :param e: Energy :math:`E`.
        :type e: float

        :return: :math:`ρ(E) = \\left|k'(E) k(E)\\right|`
        :rtype: float
        """
        d = self._dichalcogenide
        at, λ = d.at, d.λ
        return abs(2 * e - λ) * (2 * at**2)**(-1)