While we cannot define a ground-state of time-dependent quantum systems in general, the time-periodic systems can overcome this issue due to the application of Floquet theory. In principle this application results in a static-like system, so it would follow that there can be an equivalent ground-state for these time-periodic systems. Indeed we can define one from the average-energy , and in this seminar we will explain the procedures for doing so. We will also explore the properties of this ground-state and how it can be applied to develop practical calculations.C. M. Le, R. Akashi, and S. Tsuneyuki, Defining a Well-Ordered Floquet Basis by the Average Energy, Physical Review A 102, 042212 (2020).