The old quantum condition dictates that the integral of P dX over an orbit, which is the area of the circle in phase space, must be an integer multiple of the Planck constant. The area of the circle of radius √ 2 E is 2 πE. So E = n h 2 π π = n ℏ ℏ , or, in natural units where ħ = 1, the energy is an integer.