The Schrödinger equation is a partial differential equation whose solution is the wave equation, which describes the probability density of a given particle over space. The general form of the Schrödinger equation is where i is the imaginary unit, ħ is the reduced Planck constant, Ψ is the wave function, and Ĥ is the Hamiltonian operator (representing the total energy of the system). In steady-state systems (where the wave equation does not depend on time, such as in an atomic or molecular orbital) this simplifies to