Here H denotes the Hamiltonian, the observable corresponding to the total energy of the system, and ℏ ℏ is the reduced Planck constant. The constant i ℏ ℏ is introduced so that the Hamiltonian is reduced to the classical Hamiltonian in cases where the quantum system can be approximated by a classical system; the ability to make such an approximation in certain limits is called the correspondence principle.